Fix the header being included when trying to specialize std::hash

Summary: `std::hash` lives in `<functional>`, not `<xstddef>`.

Reviewed By: yfeldblum

Differential Revision: D5097468

fbshipit-source-id: d0684491d1d4b0f83f07ad837a66a915ce2ec8a6

folly/portability/PThread.h

@@ -112,7 +112,7 @@ inline int pthread_attr_getguardsize(
   return 0;
 }
 
-#include <xstddef>
+#include <functional>
 namespace std {
 template <>
 struct hash<pthread_t> {