src: Update or add auto generated license headers

Closes #55

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/169>

src/image_metadata/obj.rs

@@ -1,3 +1,20 @@
+// Copyright (c) 2022-2023 Sophie Herold
+//
+// 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-or-later
+
 use gio::prelude::*;
 
 use super::{GPSLocation, ImageMetadata, Orientation};