PM / Wakeup: Remove useless synchronize_rcu() call

wakeup_source_add() adds an item into wakeup_sources list.

There is no need to call synchronize_rcu() at this point.

Its only needed in wakeup_source_remove()

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>