Fix licenses

src/notifications/Manager.cpp

+// SPDX-FileCopyrightText: 2021 Nheko Contributors
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 #include "notifications/Manager.h"
 
 #include "Cache.h"

src/notifications/ManagerLinux.cpp

+// SPDX-FileCopyrightText: 2012 Roland Hieber <rohieb@rohieb.name>
+// SPDX-FileCopyrightText: 2021 Nheko Contributors
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 #include "notifications/Manager.h"
 
 #include <QDBusConnection>

src/notifications/ManagerMac.cpp

+// SPDX-FileCopyrightText: 2021 Nheko Contributors
+//
+// SPDX-License-Identifier: GPL-3.0-or-later
+
 #include "Manager.h"
 
 #include <QRegularExpression>