Summary:
D2872406 enables -Werror=unused-parameter, which in conjunction with mode/opt
(which eliminates asserts) causes two files in folly/io/async to fail to compile.
This change works around the error.

Reviewed By: vchalyshev

Differential Revision: D2874625

fb-gh-sync-id: 97104679f964390c5df88ee7831af7df243a152a