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

diff --git a/folly/portability/PThread.h b/folly/portability/PThread.h
index e632458189aa30a6e6d748717abf0e26281483f9..8b2f305113c49d2eba2569fb4706581620ab09e7 100755 (executable)
--- a/folly/portability/PThread.h
+++ b/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> {