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