Licensing: add header to QJsonModel copyright

- mention that this is shipped as part of Calamares
 - SPDX info

src/libcalamaresui/utils/qjsonitem.cpp

@@ -1,3 +1,9 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   SPDX-License-Identifier: GPL-3.0+
+ *   License-Filename: LICENSES/GPLv3+-QJsonModel
+ */
+
 /***********************************************
     Copyright (C) 2014  Schutz Sacha
     This file is part of QJsonModel (https://github.com/dridk/QJsonmodel).

src/libcalamaresui/utils/qjsonitem.h

@@ -1,3 +1,9 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   SPDX-License-Identifier: GPL-3.0+
+ *   License-Filename: LICENSES/GPLv3+-QJsonModel
+ */
+
 #ifndef JSONITEM_H
 #define JSONITEM_H
 #include <QtCore>

src/libcalamaresui/utils/qjsonmodel.cpp

@@ -1,3 +1,9 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   SPDX-License-Identifier: GPL-3.0+
+ *   License-Filename: LICENSES/GPLv3+-QJsonModel
+ */
+
 /***********************************************
     Copyright (C) 2014  Schutz Sacha
     This file is part of QJsonModel (https://github.com/dridk/QJsonmodel).

src/libcalamaresui/utils/qjsonmodel.h

@@ -1,3 +1,9 @@
+/* === This file is part of Calamares - <https://github.com/calamares> ===
+ *
+ *   SPDX-License-Identifier: GPL-3.0+
+ *   License-Filename: LICENSES/GPLv3+-QJsonModel
+ */
+
 #ifndef QJSONMODEL_H
 #define QJSONMODEL_H