cmake: set the CAP_WAKE_ALARM capability on the powerdevil

timerfd_create with CLOCK_REALTIME_ALARM which can wake system from
suspend/deep sleep needs executable to have a CAP_WAKE_ALARM. However
dependency is not marked as the required since if setting
CLOCK_REALTIME_ALARM fails, we fallback to CLOCK_REALTIME which can
allow setting alarms but not which can wake system from suspend.

See also: man 2 timerfd_create