MapboxGL plugin not showing up on Windows

Apparently, with https://codereview.qt-project.org/#/c/210545/ we forgot to modify QGeoServiceProviderFactoryMapboxGL::createMappingManagerEngine where the condition

#ifdef Q_OS_WIN
    if (QOpenGLContext::openGLModuleType() != QOpenGLContext::LibGLES) {
        qWarning("The Mapbox GL plugin only supports OpenGL ES on Windows. \
            Try setting the environment variable QT_OPENGL to 'angle'.");

        return 0;
    }
#endif

should now be changed, or else it won't create a manager

Note: it might be that this bug will make package testing fail, but if not, feel free to remove the blocker