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/midx.c b/midx.c
index 84d7a53..24799be 100644
--- a/midx.c
+++ b/midx.c
@@ -3,6 +3,7 @@
 #include "config.h"
 #include "csum-file.h"
 #include "dir.h"
+#include "hex.h"
 #include "lockfile.h"
 #include "packfile.h"
 #include "object-store.h"