Diffusion - fix error log spew from D5894

Summary: forgot to declare this method as static. PHP manages to make it work functionally anyway. Ref T2784. Fixes T3175.

Test Plan: loaded diffusion, observed no errors

Reviewers: chad, epriestley

CC: aran, Korvin

Maniphest Tasks: T3175, T2784

Differential Revision: https://secure.phabricator.com/D5899