taskqueue: Move the timeout_task definition to _task.h

So that timeout_task may be embedded into structures without pulling in
too many other definitions. No functional change intended.

MFC after: 1 week
Sponsored by: Klara, Inc.
Sponsored by: Stormshield

(cherry picked from commit bea256f3c5b9d9608278377576047bf0e591a4e7)