Fix glx_xcomposite build after change in QtPlatformSupport

The API was changed by qtbase's af1e32426c50694c0e4c1c292aa5eeee3b38c7bc

Change-Id: I38ec8f1be5f5ffb4b93097ce4e72c9a61c9459d0
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>