Fix warning about use of uninitialized variable

qwaylandwindow.cpp:443:25: warning: ‘transform’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Change-Id: Ic0a96cfda35d331c9957a527eea7f8b987191b5f
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>