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/notes.c b/notes.c
index f2805d5..c9e4b8b 100644
--- a/notes.c
+++ b/notes.c
@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "config.h"
+#include "hex.h"
 #include "notes.h"
 #include "object-store.h"
 #include "blob.h"