opensubtitles: Fix warnings running gnome-music
opensubtitles: Fix warnings running gnome-music

When running gnome-music, grl_source_query() will call
expand_operation_keys() to try and find which sources could help add
metadata.

https://bugzilla.gnome.org/show_bug.cgi?id=754451

GNOME/grilo-plugins,MikePetullo/grilo-plugins,MikePetullo/grilo-plugins,jasuarez/grilo-plugins,grilofw/grilo-plugins,grilofw/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,MikePetullo/grilo-plugins