qtmultimedia FTBFS on hurd due to linux-only headers, which might be avoidable

I know Hurd is not an official arch but please allow me to explain this one, I think we might get a good compromise solution.

The Qt gstreamer code related to video capture includes <linux/videodev2.h>, which is not present in Hurd.

We can simply disable gstreamer in Hurd by adding the above include in config.tests/gstreamer/main.cpp (thanks Dmitry Shachnev), but that would disable *all* the gstreamer stuff.

It would be good if (if possible) we could just disable the video stuff.

If you give me some hints I might try to do a patch myself.