folly/experimental/hazptr: fix gcc 5 build

Summary: Fix includes in memory_resource.h

Reviewed By: yfeldblum

Differential Revision: D3935848

fbshipit-source-id: 713ce85ad17072779dfa1c667459e367c5e1d4b7

folly/experimental/hazptr/memory_resource.h

@@ -15,16 +15,17 @@
  */
 #pragma once
 
-namespace folly {
-namespace hazptr {
-
 ////////////////////////////////////////////////////////////////////////////////
 /// Disclaimer: This is intended only as a partial stand-in for
 /// std::pmr::memory_resource (C++17) as needed for developing a
 /// hazptr prototype.
 ////////////////////////////////////////////////////////////////////////////////
+#include <cstddef>
 #include <memory>
 
+namespace folly {
+namespace hazptr {
+
 class memory_resource {
  public:
   virtual ~memory_resource() = default;