Base class of `differential.field-selector` should be DifferentialFieldSelector, not DifferentialDefaultFieldSelector

Summary: Missed this -- the "Default" flavor extends from the actual abstract base.

Test Plan: Loaded setup issues, no longer saw an issue raised for my local extension class.

Reviewers: codeblock

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255

Differential Revision: https://secure.phabricator.com/D4463