Fix visibility of the LinesOfALargeFile::readMore() method

Summary: Ref T6822.

Test Plan: This only has one call site (in LinesOfALarge::next()).

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11236