Fix compilation issue related to Fibers WhenN

Summary: In a separate diff, I made changes that switched include order, and suddently this file failed to compile. This fixes it.

Reviewed By: andriigrynenko

Differential Revision: D3574775

fbshipit-source-id: 02422d803708aa8490123d19aab5a16c10fdc36e

folly/fibers/WhenN.h

@@ -15,6 +15,8 @@
  */
 #pragma once
 
+#include <memory>
+
 namespace folly {
 namespace fibers {