spotify: make common module public

Karapulse implements its own bin wrapping spotifyaudiosrc, spotifylyricssrc and videotestsrc for rendering Spotify songs.

This element has lived in my own gst-plugins-rs branch for ages but now that spotifylyricssrc has been usptreamed I'd like to move it to the Karapulse source tree. But to be able to do so it needs to be able to use the common module from the spotify plugin.