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/cache-tree.c b/cache-tree.c
index 256f98c..9d46ece 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -1,5 +1,6 @@
 #include "git-compat-util.h"
 #include "alloc.h"
+#include "hex.h"
 #include "lockfile.h"
 #include "tree.h"
 #include "tree-walk.h"