codgen: generate impl T if there are builder properties as well
codgen: generate impl T if there are builder properties as well

Fixes https://github.com/gtk-rs/gtk4-rs/issues/405
gtk-rs/gir,gtk-rs/gir,GuillaumeGomez/gir,GuillaumeGomez/gir,gtk-rs/gir,GuillaumeGomez/gir