Fix Qt key extension loading

A typo in QWaylandDisplay meant that the Qt key extension was not being
registered in the client.

Change-Id: I5f276beed41502cb90d4090e553415275ee5a804
Reviewed-by: Jan Arne Petersen <jan.petersen@kdab.com>
Reviewed-by: Andy Nichols <andy.nichols@digia.com>