glib: glib-macros: genum_derive: add missing copyright header
+// Copyright 2020, The Gtk-rs Project Developers.
+// See the COPYRIGHT file at the top-level directory of this distribution.
+// Licensed under the MIT license, see the LICENSE file or <http://opensource.org/licenses/MIT>
+
use anyhow::{bail, Result};
use heck::{CamelCase, KebabCase, SnakeCase};
use itertools::Itertools;
