Add some const and remove some unnecessary headers.
Suggested by Flexelint.

lib/zip_close.c

diff --git a/lib/zip_close.c b/lib/zip_close.c
index fe83fc2..a885527 100644
--- a/lib/zip_close.c
+++ b/lib/zip_close.c
@@ -562,7 +562,7 @@
 
 
 int
-_zip_changed(struct zip *za, zip_uint64_t *survivorsp)
+_zip_changed(const struct zip *za, zip_uint64_t *survivorsp)
 {
     int changed;
     zip_uint64_t i, survivors;

lib/zip_dirent.c

diff --git a/lib/zip_dirent.c b/lib/zip_dirent.c
index 595b524..db6141c 100644
--- a/lib/zip_dirent.c
+++ b/lib/zip_dirent.c
@@ -44,7 +44,7 @@
 
 static time_t _zip_d2u_time(zip_uint16_t, zip_uint16_t);
 static struct zip_string *_zip_read_string(const unsigned char **, FILE *, zip_uint16_t, int, struct zip_error *);
-static struct zip_string *_zip_dirent_process_ef_utf_8(struct zip_dirent *, zip_uint16_t, struct zip_string *);
+static struct zip_string *_zip_dirent_process_ef_utf_8(const struct zip_dirent *, zip_uint16_t, struct zip_string *);
 static struct zip_extra_field *_zip_ef_utf8(zip_uint16_t, struct zip_string *, struct zip_error *);
 
 
@@ -508,7 +508,7 @@
 
 
 static struct zip_string *
-_zip_dirent_process_ef_utf_8(struct zip_dirent *de, zip_uint16_t id, struct zip_string *str)
+_zip_dirent_process_ef_utf_8(const struct zip_dirent *de, zip_uint16_t id, struct zip_string *str)
 {
     zip_uint16_t ef_len;
     zip_uint32_t ef_crc;

lib/zip_file_get_offset.c

diff --git a/lib/zip_file_get_offset.c b/lib/zip_file_get_offset.c
index a19e906..65a011f 100644
--- a/lib/zip_file_get_offset.c
+++ b/lib/zip_file_get_offset.c
@@ -51,7 +51,7 @@
 */
 
 zip_uint64_t
-_zip_file_get_offset(struct zip *za, zip_uint64_t idx, struct zip_error *error)
+_zip_file_get_offset(const struct zip *za, zip_uint64_t idx, struct zip_error *error)
 {
     zip_uint64_t offset;
     zip_int32_t size;

lib/zip_file_set_comment.c

diff --git a/lib/zip_file_set_comment.c b/lib/zip_file_set_comment.c
index e7f9590..722f3d4 100644
--- a/lib/zip_file_set_comment.c
+++ b/lib/zip_file_set_comment.c
@@ -34,7 +34,6 @@
 
 
 #include <stdlib.h>
-#include <string.h>
 
 #include "zipint.h"
 

lib/zip_open.c

diff --git a/lib/zip_open.c b/lib/zip_open.c
index b7ddc16..e6a30d5 100644
--- a/lib/zip_open.c
+++ b/lib/zip_open.c
@@ -42,20 +42,20 @@
 
 #include "zipint.h"
 
-static void set_error(int *, struct zip_error *, int);
+static void set_error(int *, const struct zip_error *, int);
 static struct zip *_zip_allocate_new(const char *, unsigned int, int *);
 static zip_int64_t _zip_checkcons(FILE *, struct zip_cdir *, struct zip_error *);
 static void _zip_check_torrentzip(struct zip *, const struct zip_cdir *);
 static struct zip_cdir *_zip_find_central_dir(FILE *, unsigned int, int *, off_t);
 static int _zip_file_exists(const char *, unsigned int, int *);
-static int _zip_headercomp(struct zip_dirent *, struct zip_dirent *);
+static int _zip_headercomp(const struct zip_dirent *, const struct zip_dirent *);
 static unsigned char *_zip_memmem(const unsigned char *, size_t,
 				  const unsigned char *, size_t);
-static struct zip_cdir *_zip_readcdir(FILE *, off_t, unsigned char *, unsigned char *,
+static struct zip_cdir *_zip_readcdir(FILE *, off_t, unsigned char *, const unsigned char *,
 				 size_t, unsigned int, struct zip_error *);
-static struct zip_cdir *_zip_read_eocd(const unsigned char *, unsigned char *, off_t,
+static struct zip_cdir *_zip_read_eocd(const unsigned char *, const unsigned char *, off_t,
 				       size_t, unsigned int, struct zip_error *);
-static struct zip_cdir *_zip_read_eocd64(FILE *, const unsigned char *, unsigned char *,
+static struct zip_cdir *_zip_read_eocd64(FILE *, const unsigned char *, const unsigned char *,
 					 off_t, size_t, unsigned int, struct zip_error *);
 
 
@@ -175,7 +175,7 @@
 
 
 static void
-set_error(int *zep, struct zip_error *err, int ze)
+set_error(int *zep, const struct zip_error *err, int ze)
 {
     int se;
 
@@ -198,7 +198,7 @@
    entries, or NULL if unsuccessful. */
 
 static struct zip_cdir *
-_zip_readcdir(FILE *fp, off_t buf_offset, unsigned char *buf, unsigned char *eocd, size_t buflen,
+_zip_readcdir(FILE *fp, off_t buf_offset, unsigned char *buf, const unsigned char *eocd, size_t buflen,
 	      unsigned int flags, struct zip_error *error)
 {
     struct zip_cdir *cd;
@@ -402,7 +402,7 @@
    Return 0 if they are consistent, -1 if not. */
 
 static int
-_zip_headercomp(struct zip_dirent *central, struct zip_dirent *local)
+_zip_headercomp(const struct zip_dirent *central, const struct zip_dirent *local)
 {
     if ((central->version_needed != local->version_needed)
 #if 0
@@ -590,7 +590,7 @@
 
 
 static struct zip_cdir *
-_zip_read_eocd(const unsigned char *eocd, unsigned char *buf, off_t buf_offset, size_t buflen,
+_zip_read_eocd(const unsigned char *eocd, const unsigned char *buf, off_t buf_offset, size_t buflen,
 	       unsigned int flags, struct zip_error *error)
 {
     struct zip_cdir *cd;
@@ -645,7 +645,7 @@
 
 
 static struct zip_cdir *
-_zip_read_eocd64(FILE *f, const zip_uint8_t *eocd64loc, zip_uint8_t *buf,
+_zip_read_eocd64(FILE *f, const zip_uint8_t *eocd64loc, const zip_uint8_t *buf,
 		 off_t buf_offset, size_t buflen, unsigned int flags, struct zip_error *error)
 {
     struct zip_cdir *cd;

lib/zip_set_file_comment.c

diff --git a/lib/zip_set_file_comment.c b/lib/zip_set_file_comment.c
index 9643ab0..7acd0eb 100644
--- a/lib/zip_set_file_comment.c
+++ b/lib/zip_set_file_comment.c
@@ -34,7 +34,6 @@
 
 
 #include <stdlib.h>
-#include <string.h>
 
 #define _ZIP_COMPILING_DEPRECATED
 #include "zipint.h"

lib/zipint.h

diff --git a/lib/zipint.h b/lib/zipint.h
index aa9dee7..6ff370a 100644
--- a/lib/zipint.h
+++ b/lib/zipint.h
@@ -433,7 +433,7 @@
 
 int _zip_file_extra_field_prepare_for_change(struct zip *, zip_uint64_t);
 int _zip_file_fillbuf(void *, size_t, struct zip_file *);
-zip_uint64_t _zip_file_get_offset(struct zip *, zip_uint64_t, struct zip_error *);
+zip_uint64_t _zip_file_get_offset(const struct zip *, zip_uint64_t, struct zip_error *);
 
 int _zip_filerange_crc(FILE *, off_t, off_t, uLong *, struct zip_error *);
 
@@ -462,7 +462,7 @@
 struct zip_string *_zip_string_new(const zip_uint8_t *, zip_uint16_t, zip_flags_t, struct zip_error *);
 void _zip_string_write(const struct zip_string *, FILE *);
 
-int _zip_changed(struct zip *, zip_uint64_t *);
+int _zip_changed(const struct zip *, zip_uint64_t *);
 const char *_zip_get_name(struct zip *, zip_uint64_t, zip_flags_t, struct zip_error *);
 int _zip_local_header_read(struct zip *, int);
 void *_zip_memdup(const void *, size_t, struct zip_error *);