Use zip_XXX_t instead of struct zip_XXX or enum zip_XXX.

lib/zip_fopen.c

diff --git a/lib/zip_fopen.c b/lib/zip_fopen.c
index ad7c416..3adb5de 100644
--- a/lib/zip_fopen.c
+++ b/lib/zip_fopen.c
@@ -35,8 +35,8 @@
 #include "zipint.h"
 
 
-ZIP_EXTERN struct zip_file *
-zip_fopen(struct zip *za, const char *fname, zip_flags_t flags)
+ZIP_EXTERN zip_file_t *
+zip_fopen(zip_t *za, const char *fname, zip_flags_t flags)
 {
     zip_int64_t idx;