object-file.h: move declarations for object-file.c functions from cache.h

Signed-off-by: Elijah Newren <newren@gmail.com>
Acked-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/diff.c b/diff.c
index 9e6ad94..f8e0d3b 100644
--- a/diff.c
+++ b/diff.c
@@ -35,6 +35,7 @@
 #include "help.h"
 #include "promisor-remote.h"
 #include "dir.h"
+#include "object-file.h"
 #include "object-name.h"
 #include "setup.h"
 #include "strmap.h"