Fix visibility of the LiskDAO::willWriteData method
Summary: Ref T6822.
Test Plan: grep. This method is only called from LiskDAO::update().
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, epriestley
Maniphest Tasks: T6822
Differential Revision: https://secure.phabricator.com/D11409