audio/shortwave: unbreak with libadwaita >= 2021-05-20 snapshot

$ shortwave
(de.haeckerfelix.Shortwave:1234): GLib-GObject-WARNING : specified class size for type 'SwSongRow' is smaller than the parent type's 'AdwActionRow' class size
(de.haeckerfelix.Shortwave:1234): GLib-GObject-WARNING : cannot add private field to invalid (non-instantiatable) type '<invalid>'
thread 'main' panicked at 'assertion failed: type_.is_valid()', src/ui/song_row.rs:49:5
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace