Summary:
[Folly] Fix up fiber-related guards in futures code. Because of cyclic `#include`s the preprocessor symbol definition must be moved.

Fixes #1061.

Reviewed By: wez, andriigrynenko

Differential Revision: D14555070

fbshipit-source-id: 5e4966fc0f98351924c00a459cc2d8893df4b368