dbUtils: return multiple tag values per song

When collecting tag values for the result set, add all of a song's tag
values of the searched type.  This affects the "list" command.
Previously, "list" only considered the first tag value of a song.

src/dbUtils.c

@@ -246,7 +246,6 @@ visitTag(struct client *client, struct strset *set,
 	for (unsigned i = 0; i < tag->num_items; i++) {
 		if (tag->items[i]->type == tagType) {
 			strset_add(set, tag->items[i]->value);
-			return;
 		}
 	}