codegen/enums: Add missing trait_impls::generate call
codegen/enums: Add missing trait_impls::generate call

This function is responsible for generating a Display impl, and caused
unused std::fmt import previously.

gtk-rs/gir,GuillaumeGomez/gir,GuillaumeGomez/gir,gtk-rs/gir,gtk-rs/gir,GuillaumeGomez/gir