gpodder: add mutagen to propagatedBuildInputs

Mutagen is an optional dependency, but is needed by the tagging extension. If
it's missing, the extension won't work and you'll get a stack trace about it
every time you run gpodder.

pkgs/applications/audio/gpodder/default.nix

@@ -63,6 +63,7 @@ python3Packages.buildPythonApplication rec {
     eyeD3
     podcastparser
     html5lib
+    mutagen
   ];
 
   makeFlags = [