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/apply.c b/apply.c
index e6dbab2..5b44286 100644
--- a/apply.c
+++ b/apply.c
@@ -8,6 +8,7 @@
  */
 
 #include "cache.h"
+#include "config.h"
 #include "blob.h"
 #include "delta.h"
 #include "diff.h"