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/builtin/index-pack.c b/builtin/index-pack.c
index 7e4b69f..21c0e10 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -2,6 +2,7 @@
 #include "alloc.h"
 #include "config.h"
 #include "delta.h"
+#include "hex.h"
 #include "pack.h"
 #include "csum-file.h"
 #include "blob.h"