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>

archive.c

@@ -500,7 +500,7 @@ static void parse_treeish_arg(const char **argv,
 	ar_args->time = archive_time;
 }
 
-static void extra_file_info_clear(void *util, const char *str)
+static void extra_file_info_clear(void *util, const char *str UNUSED)
 {
 	struct extra_file_info *info = util;
 	free(info->base);