Enable the spandsp plugin
    
    Olivier Crête said that having the spandsp plugin in Fedora's
    gstreamer1-plugins-bad-free would enable running the farstream02 unit
    tests. This seems like a worthwhile goal to pursue.
    
    However, this does introduce a new dependency on the spandsp package,
    and since it's only being enabled to make upstream development easier,
    it's better to keep it in the -extras sub-package for the time being.