GPS: change license to GPLv3

src/main.rs

@@ -1,3 +1,22 @@
+// main.rs
+//
+// Copyright 2021 Tom A. Wagner <tom.a.wagner@protonmail.com>
+// Copyright 2021 Stéphane Cerveau <scerveau@collabora.com>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+//
+// SPDX-License-Identifier: GPL-3.0-only
 mod mainwindow;
 
 use gtk::prelude::*;