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/url.c b/url.c
index e04bd60..bf318c0 100644
--- a/url.c
+++ b/url.c
@@ -1,4 +1,5 @@
 #include "cache.h"
+#include "hex.h"
 #include "url.h"
 
 int is_urlschemechar(int first_flag, int ch)