Relicense as LicenseRef-KDE-Accepted-GPL

@mbruchert was asked and approved.

src/example/main.cpp

@@ -1,6 +1,6 @@
 // SPDX-FileCopyrightText: 2021 Jonah Brüchert <[email protected]>
 //
-// SPDX-License-Identifier: GPL-2.0-or-later
+// SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
 
 #include <ytmusic.h>