Don't build winrtrunner on other platforms than WinRT

It isn't really useful unless your target is WinRT, in which case you
can build winrtrunner with that version of Qt.

Change-Id: Ifc2f80b0b8974e1d661894dac19d33b55f13ed7f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>