cache.h: remove dependence on hex.h; make other files include it explicitly

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/object-file.c b/object-file.c
index 18d6522..43a39dd 100644
--- a/object-file.c
+++ b/object-file.c
@@ -9,6 +9,7 @@
 #include "git-compat-util.h"
 #include "alloc.h"
 #include "config.h"
+#include "hex.h"
 #include "string-list.h"
 #include "lockfile.h"
 #include "delta.h"