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/http-fetch.c b/http-fetch.c
index 258fec2..8db35b9 100644
--- a/http-fetch.c
+++ b/http-fetch.c
@@ -1,6 +1,7 @@
 #include "cache.h"
 #include "config.h"
 #include "exec-cmd.h"
+#include "hex.h"
 #include "http.h"
 #include "walker.h"
 #include "strvec.h"