Move folly/SmallLocks.h to folly/synchronization/
Move folly/SmallLocks.h to folly/synchronization/

Summary: [Folly] Move `folly/SmallLocks.h` to `folly/synchronization/`.

Differential Revision: D6806563

fbshipit-source-id: 26c7be6b6a01673a4f58a0952b5c531528f8ca7f

facebook/wangle,facebook/wangle,facebook/wangle