Fix warning caught by Clang on Linux

qglxintegration.h:78:17: error: private field 'm_screen' is not used [-Werror,-Wunused-private-field]

Change-Id: I7ffeda1dd4be962a4bfb43ebd3aa1dbd969a9837
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>