Change LiskDAO::generatePHID to be public

Summary: Ref T6822. There are a bunch of places where we call `$something->generatePHID(...)` externally (outside of the class). Therefore, these methods need to be `public`.

Test Plan: I wouldn't expect //increasing// method visibility to break anything.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11363

src/infrastructure/storage/lisk/LiskDAO.php

@@ -1291,7 +1291,7 @@ abstract class LiskDAO {
    *
    * @task   hook
    */
-  protected function generatePHID() {
+  public function generatePHID() {
     throw new Exception(
       'To use CONFIG_AUX_PHID, you need to overload '.
       'generatePHID() to perform PHID generation.');