clementine: disable unfree dependencies by default

Clementine has an optional dependency on libspotify, which is unfree.
Enabling libspotify unconditionally prevented Hydra from distributing
Clementine. Now, we optionally enable it based on
config.clementine.spotify.
Merge pull request #2740 from ttuegel/clementine

clementine: disable unfree dependencies by default