Fix visibility of the PhutilOAuth1Future::getProxiedFuture() method
Summary: Ref T6822.
Test Plan: grep
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T6822
Differential Revision: https://secure.phabricator.com/D11408