Drop a dead include of FBString.h from MemoryMapping.h
Drop a dead include of FBString.h from MemoryMapping.h

Summary: MemoryMapping doesn't use fbstring directly at all, so this include isn't needed.

Reviewed By: yfeldblum

Differential Revision: D5188209

fbshipit-source-id: d86b876a96343c830295bea1162580215a340815

Orvid/folly,facebook/folly,Orvid/folly,Orvid/folly,Orvid/folly,rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,facebook/folly,rklabs/folly,Orvid/folly,facebook/folly,rklabs/folly,rklabs/folly