Add missing include in folly/memory/ReentrantAllocator.h

Summary:
[Folly] Add missing `#include <memory>` in `folly/memory/ReentrantAllocator.h`.

Closes: https://github.com/facebook/folly/pull/1379.

Reviewed By: Orvid

Differential Revision: D22062251

fbshipit-source-id: 30e2f5aa2708048c478a298cf7e12a012ba59540

folly/memory/ReentrantAllocator.h

@@ -19,6 +19,7 @@
 #include <atomic>
 #include <cstddef>
 #include <limits>
+#include <memory>
 #include <type_traits>
 
 namespace folly {