Add missing define guard for folly-config.h

Summary:
Other inclusions of folly-config.h watch for this.
It must have been missed when this file was added.

Test Plan: Fixes Mac OSX build.

Reviewed By: seanc@fb.com

FB internal diff: D1028354

folly/detail/Clock.h

@@ -20,7 +20,9 @@
 #include <ctime>
 #include <cstdint>
 
+#ifndef FOLLY_NO_CONFIG
 #include "folly/folly-config.h"
+#endif
 
 #if FOLLY_HAVE_CLOCK_GETTIME
 #error This should only be used as a workaround for platforms \