meson: Re-instate mediaLib support
meson: Re-instate mediaLib support

We dropped it with the Meson build port, and I'm not entirely sure it's
still tested, but it doesn't cost us anything to re-enable the check.

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf