libalpm: mark filelist_contains as taking a const filelist

This is useful for bindings as it guarantees the value will not be
changed.
Signed-off-by: Allan McRae <allan@archlinux.org>

lib/libalpm/alpm.h

@@ -156,7 +156,7 @@ typedef struct _alpm_backup_t {
  * @param path the path to search for in the package
  * @return a pointer to the matching file or NULL if not found
  */
-alpm_file_t *alpm_filelist_contains(alpm_filelist_t *filelist, const char *path);
+alpm_file_t *alpm_filelist_contains(const alpm_filelist_t *filelist, const char *path);
 
 /* End of libalpm_files */
 /** @} */

lib/libalpm/filelist.c

@@ -118,7 +118,7 @@ static int _alpm_files_cmp(const void *f1, const void *f2)
 	return strcmp(file1->name, file2->name);
 }
 
-alpm_file_t SYMEXPORT *alpm_filelist_contains(alpm_filelist_t *filelist,
+alpm_file_t SYMEXPORT *alpm_filelist_contains(const alpm_filelist_t *filelist,
 		const char *path)
 {
 	alpm_file_t key;