docs: normalize source license headers to gpl2 only

include/nwqueue.h

@@ -1,3 +1,19 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * This file is part of mars_nwe.
+ *
+ * Copyright (C) 1993-2000 Martin Stover, Marburg, Germany
+ * Copyright (C) 2026 Mario Fetka
+ *
+ * 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; version 2 only.
+ *
+ * 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.
+ */
 /* nwqueue.h 14-Apr-98 */
 #ifndef _NWQUEUE_H_
 #define _NWQUEUE_H_