Add missing includes and forward declarations

I looped over the toplevel header files, creating a temporary two-line C
program for each consisting of
  #include "git-compat-util.h"
  #include $HEADER
This patch is the result of manually fixing errors in compiling those
tiny programs.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-objects.h

@@ -2,6 +2,7 @@
 #define PACK_OBJECTS_H
 
 #include "object-store.h"
+#include "pack.h"
 
 #define DEFAULT_DELTA_CACHE_SIZE (256 * 1024 * 1024)