folly: unconditionally include boost mutex definitions on macOS

Summary:
macOS doesn't have timedwait functionality and this header file was
gating including the boost headers on that check which caused dependent modules
to fail to compile because `boost::mutex` and `boost::recursive_mutex` were not
known to the compiler.

Reviewed By: yfeldblum

Differential Revision: D3585470

fbshipit-source-id: 7f8d9603e95ce01328103c7c6ac0bc75a35ddf4d

folly/LockTraitsBoost.h

@@ -22,11 +22,11 @@
  */
 #pragma once
 
+#include <boost/thread.hpp>
 #include <folly/LockTraits.h>
 
 #if FOLLY_LOCK_TRAITS_HAVE_TIMED_MUTEXES
 
-#include <boost/thread.hpp>
 
 namespace folly {