Fix a folly build failure with GCC5
Fix a folly build failure with GCC5

Summary:
I tested it doesn't break the gcc 4.9 and clang builds - I didn't try any other versions of anything.

build[all]

Reviewed By: meyering

Differential Revision: D3757152

fbshipit-source-id: a126cd31cb89057dececfed1b265f452ee29fbff

rklabs/folly,floxard/folly,facebook/folly,facebook/folly,charsyam/folly,floxard/folly,rklabs/folly,floxard/folly,facebook/folly,facebook/folly,charsyam/folly,Orvid/folly,rklabs/folly,Orvid/folly,Orvid/folly,Orvid/folly,charsyam/folly,floxard/folly,charsyam/folly,Orvid/folly,rklabs/folly,floxard/folly,facebook/folly,rklabs/folly