Fix build of compositors by always including QOpenGLContext.

QOpenGLContext is always required even when QT_COMPOSITOR_WAYLAND_GL is
not defined.  Moving the include outside the ifdef makes compositors
build without problems.

Change-Id: I4b8903b06a5f58ad8fd2953dbde1476877683cb4
Reviewed-by: Laszlo Agocs <laszlo.p.agocs@nokia.com>