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/refspec.c b/refspec.c
index ec336ec..28d9091 100644
--- a/refspec.c
+++ b/refspec.c
@@ -1,5 +1,6 @@
 #include "git-compat-util.h"
 #include "alloc.h"
+#include "hex.h"
 #include "strvec.h"
 #include "refs.h"
 #include "refspec.h"