Address licence of plugin.h

The rest of the project is GPLv2, this one is GPLv3 for no apparent
reason. Even the .cpp file is GPLv2.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>

src/plugins/private/plugin.h

@@ -1,7 +1,7 @@
 /*
     SPDX-FileCopyrightText: 2021 Vlad Zahorodnii <vlad.zahorodnii@kde.org>
 
-    SPDX-License-Identifier: GPL-3.0-or-later
+    SPDX-License-Identifier: GPL-2.0-or-later
 */
 
 #pragma once