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/patch-ids.c b/patch-ids.c
index 3153446..a4473a8 100644
--- a/patch-ids.c
+++ b/patch-ids.c
@@ -2,6 +2,7 @@
 #include "diff.h"
 #include "commit.h"
 #include "hash-lookup.h"
+#include "hex.h"
 #include "patch-ids.h"
 
 static int patch_id_defined(struct commit *commit)