fix qm output dir for non-prefix builds

Task-number: QTBUG-31727
Change-Id: I696a7423beffb3114ac6f6b2a5dcbb297801c5a0
Reviewed-by: Eike Ziller <eike.ziller@digia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>