Countdown: fix PhutilMissingSymbolException

Summary:
A previous change added a couple of Countdown-related classes:

7ed35123a347a05c70c97eba2bec2b36eb2b3218

This change updates the related library map (arc liberate) in order to
fix any related regression (like PhutilMissingSymbolException).

Closes T15271

Test Plan:

• you run arc liberate and the file does not change
• you do not see any PhutilMissingSymbolException exception

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15271

Differential Revision: https://we.phorge.it/D25133