sparse: Fix an "symbol 'cmd_index_pack' not declared" warning

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

builtin/index-pack.c

-#include "cache.h"
+#include "builtin.h"
 #include "delta.h"
 #include "pack.h"
 #include "csum-file.h"