CoreWLan: wait for scan thread to finish

If the scan thread is running when QCoreWlanEngine is destroyed
it will access stale data and cause a crash.

Task-number: QTBUG-36000
Change-Id: I8cc9e39a3f7d4736da39e8b31f6963db35318f19
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>