gstreamer: Always build dmabuf support

I just spent an hour trying to figure out why things don't work. And it was an optional dependency hidden 3 layers deep in some meson file.

This really has to stop.

And because just like in GTK, GStreamer's dmabuf APIs are always available (they will just fail on Windows etc), there's no need to have any conditions.

The only difference is that the GStreamer media backend now requires GStreamer 1.24.