object-name.h: move declarations for object-name.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-lib.c b/diff-lib.c
index 8b5cca9..d292405 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -8,6 +8,7 @@
 #include "diffcore.h"
 #include "gettext.h"
 #include "hex.h"
+#include "object-name.h"
 #include "revision.h"
 #include "cache-tree.h"
 #include "unpack-trees.h"