Add missing header in WTCallback.h
Add missing header in WTCallback.h

Summary: Add definition to EventBase.

Differential Revision: D23004586

fbshipit-source-id: 828767b3d9a43c33bcf047b9b06c08bfba8fde68

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly