Convert one more file to SPDX license headers that was previously missed

plugins/plasma/pimeventsplugin/settingschangenotifier.cpp

 /*
- * Copyright (C) 2016  Daniel Vrátil <dvratil@kde.org>
+ * SPDX-FileCopyrightText: 2016 Daniel Vrátil <dvratil@kde.org>
  *
- * 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 2 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, write to the Free Software Foundation, Inc.,
- * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ * SPDX-License-Identifier: GPL-2.0-or-later
  *
  */