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