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/refs/files-backend.c b/refs/files-backend.c
index b899543..6f257c7 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1,5 +1,6 @@
 #include "../cache.h"
 #include "../config.h"
+#include "../hex.h"
 #include "../refs.h"
 #include "refs-internal.h"
 #include "ref-cache.h"