locate: add "pure" attributes

src/locate.c

@@ -104,6 +104,7 @@ locate_item_list_parse(char *argv[], int argc, bool fold_case)
 	return list;
 }
 
+gcc_pure
 static bool
 locate_tag_search(const struct song *song, enum tag_type type, const char *str)
 {
@@ -164,6 +165,7 @@ locate_song_search(const struct song *song,
 	return true;
 }
 
+gcc_pure
 static bool
 locate_tag_match(const struct song *song, enum tag_type type, const char *str)
 {

src/locate.h

@@ -48,6 +48,7 @@ struct locate_item_list {
 	struct locate_item items[1];
 };
 
+gcc_pure
 int
 locate_parse_type(const char *str);
 
@@ -67,11 +68,13 @@ gcc_nonnull(1)
 void
 locate_item_list_free(struct locate_item_list *list);
 
+gcc_pure
 gcc_nonnull(1,2)
 bool
 locate_song_search(const struct song *song,
 		   const struct locate_item_list *criteria);
 
+gcc_pure
 gcc_nonnull(1,2)
 bool
 locate_song_match(const struct song *song,