Fix FBString under MSVC
Fix FBString under MSVC

Summary: MSVC requires the declaration to match a bit closer than GCC does.

Reviewed By: yfeldblum, ot

Differential Revision: D4981404

fbshipit-source-id: 92ee40c40f66725c09a3087e49d99ebae222c2f2

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