test-string-list.c: Fix some sparse warnings

In particular, sparse complains as follows:

        SP test-string-list.c
    test-string-list.c:10:6: warning: symbol 'parse_string_list' was not \
        declared. Should it be static?
    test-string-list.c:18:6: warning: symbol 'write_list' was not \
        declared. Should it be static?
    test-string-list.c:25:6: warning: symbol 'write_list_compact' was not \
        declared. Should it be static?
    test-string-list.c:38:5: warning: symbol 'prefix_cb' was not \
        declared. Should it be static?

In order to suppress the warnings, since the above symbols do not
need more than file scope, we simply include the static modifier
in their declaration.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-string-list.c

@@ -7,22 +7,22 @@
  * list (as opposed to "", which indicates a string list containing a
  * single empty string).  list->strdup_strings must be set.
  */
-void parse_string_list(struct string_list *list, const char *arg)
+static void parse_string_list(struct string_list *list, const char *arg)
 {
 	if (!strcmp(arg, "-"))
 		return;
 
 	(void)string_list_split(list, arg, ':', -1);
 }
 
-void write_list(const struct string_list *list)
+static void write_list(const struct string_list *list)
 {
 	int i;
 	for (i = 0; i < list->nr; i++)
 		printf("[%d]: \"%s\"\n", i, list->items[i].string);
 }
 
-void write_list_compact(const struct string_list *list)
+static void write_list_compact(const struct string_list *list)
 {
 	int i;
 	if (!list->nr)
@@ -35,7 +35,7 @@ void write_list_compact(const struct string_list *list)
 	}
 }
 
-int prefix_cb(struct string_list_item *item, void *cb_data)
+static int prefix_cb(struct string_list_item *item, void *cb_data)
 {
 	const char *prefix = (const char *)cb_data;
 	return !prefixcmp(item->string, prefix);