string-list: mark unused callback parameters

String-lists may be used with callbacks for clearing or iteration. These
callbacks need to conform to a particular interface, even though not
every callback needs all of its parameters. Mark the unused ones to make
-Wunused-parameter happy.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

string-list.c

@@ -156,7 +156,7 @@ void filter_string_list(struct string_list *list, int free_util,
 	list->nr = dst;
 }
 
-static int item_is_not_empty(struct string_list_item *item, void *unused)
+static int item_is_not_empty(struct string_list_item *item, void *data UNUSED)
 {
 	return *item->string != '\0';
 }