Remove unused dlfcn.h include from thrift/lib/cpp2/Flags.h
Remove unused dlfcn.h include from thrift/lib/cpp2/Flags.h

Summary:
It seems that no functions from this header are currently in use in this file.
This include is causing failure in windows:
https://www.internalfb.com/intern/sandcastle/job/22517998441155002/

Reviewed By: iahs

Differential Revision: D22756577

fbshipit-source-id: f8d70e0062697bbd6df55d1e01547dae5c45e791

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift