Fix autotools build afte removing folly/Optional.cpp
Fix autotools build afte removing folly/Optional.cpp

Summary: [Folly] Fix autotools build afte removing `folly/Optional.cpp`.

Reviewed By: akrieger

Differential Revision: D7737170

fbshipit-source-id: 1a9286f240bf5a15ee08b707b69377f8d3cfdb48

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly