Update license statements to the SPDX tags

src/libdanbooru/libdanbooru_debug.h

@@ -1,20 +1,9 @@
 /*
- * Copyright 2015 Luca Beltrame <lbeltrame@kde.org>
+ * SPDX-FileCopyrightText: 2015 Luca Beltrame <lbeltrame@kde.org>
  *
  * This file is part of Danbooru Client.
  *
- * Danbooru Client 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.
- *
- * Danbooru Client 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 Danbooru Client. If not, see <http://www.gnu.org/licenses/>.
+ *SPDX-License-Identifier: GPL-3.0-or-later
  */
 
 #ifndef DANBOORU_CLIENT_DEBUG_H