Re-fix the warning from cached-powers.cc

This file is now getting included in a different module. See also
e02cb2b31ab0b171f11d278305d9f532f005bc80,
60aed669345be33b916c44556555b922aa3ed928, and
66e72c2cf08c2d2c381c6c22e4760d0df758af52.

Change-Id: Ic886bdf823d5dd6d9ae3df64608d8bb9d901661f
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>