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.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1907>

audio/spotify/src/lib.rs

@@ -14,7 +14,7 @@
  */
 use gst::glib;
 
-mod common;
+pub mod common;
 mod spotifyaudiosrc;
 mod spotifylyricssrc;