send-pack.c: mark a file-local function static

Commit f2c681c ("send-pack: support pushing from a shallow clone
via http", 05-12-2013) adds the 'advertise_shallow_grafts_buf'
function as an external symbol.

Noticed by sparse. ("'advertise_shallow_grafts_buf' was not declared.
Should it be static?")

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

send-pack.c

@@ -183,7 +183,7 @@ static int advertise_shallow_grafts_cb(const struct commit_graft *graft, void *c
 	return 0;
 }
 
-void advertise_shallow_grafts_buf(struct strbuf *sb)
+static void advertise_shallow_grafts_buf(struct strbuf *sb)
 {
 	if (!is_repository_shallow())
 		return;