Use zip_XXX_t instead of struct zip_XXX or enum zip_XXX.

lib/zip_fopen_index.c

diff --git a/lib/zip_fopen_index.c b/lib/zip_fopen_index.c
index 7887eef..e9a169a 100644
--- a/lib/zip_fopen_index.c
+++ b/lib/zip_fopen_index.c
@@ -39,8 +39,8 @@
 #include "zipint.h"
 
 
-ZIP_EXTERN struct zip_file *
-zip_fopen_index(struct zip *za, zip_uint64_t index, zip_flags_t flags)
+ZIP_EXTERN zip_file_t *
+zip_fopen_index(zip_t *za, zip_uint64_t index, zip_flags_t flags)
 {
     return zip_fopen_index_encrypted(za, index, flags, za->default_password);
 }