Use #if !FOLLY_MOBILE instead of #ifndef FOLLY_MOBILE
Use #if !FOLLY_MOBILE instead of #ifndef FOLLY_MOBILE

Summary: Use #if !FOLLY_MOBILE instead of #ifndef FOLLY_MOBILE

Reviewed By: nbronson

Differential Revision: D16521806

fbshipit-source-id: ae9682a1456d6528820b4b6bb290816cc39ac8de

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst