config: don't include config.h by default

Stop including config.h by default in cache.h.  Instead only include
config.h in those files which require use of the config system.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/reset.c b/builtin/reset.c
index fc3b906..fdd5e5e 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -8,6 +8,7 @@
  * Copyright (c) 2005, 2006 Linus Torvalds and Junio C Hamano
  */
 #include "builtin.h"
+#include "config.h"
 #include "lockfile.h"
 #include "tag.h"
 #include "object.h"