Compilation fails for qtwayland if QT_NO_OPENGL is defined

If the qtwayland qpa plugin is compiled without OpenGL support, compilation will fail in the client module with:

In file included from qwaylandshmbackingstore.cpp:41:0:
qwaylandshmbackingstore_p.h:96:12: error: 'QImage QWaylandShmBackingStore::toImage() const' marked override, but does not override
QImage toImage() const Q_DECL_OVERRIDE;

This is due to QPlatformBackingStore::toImage() being guarded by a #ifndef QT_NO_OPENGL but QWaylandShmBackingStore::toImage() assuming OpenGL support.

The same problem also exists for QWaylandIntegration::createPlatformOpenGLContext (see patch for more information).

Adding #ifndef QT_NO_OPENGL guards fixes the compilation.