object-store.h: reduce unnecessary includes

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

object-store.h

@@ -2,16 +2,16 @@
 #define OBJECT_STORE_H
 
 #include "object.h"
-#include "oidmap.h"
 #include "list.h"
-#include "oid-array.h"
-#include "strbuf.h"
 #include "thread-utils.h"
 #include "khash.h"
 #include "dir.h"
-#include "oidtree.h"
 #include "oidset.h"
 
+struct oidmap;
+struct oidtree;
+struct strbuf;
+
 struct object_directory {
 	struct object_directory *next;