[libcalamaresui] Fix build

 - Include all headers for types that need to be fully-defined
   (e.g. return types). This guards against uses in contexts where
   those headers have not been implicitly or previously included.

FIXES #948

src/libcalamaresui/viewpages/PythonQtGlobalStorageWrapper.h

 /* === This file is part of Calamares - <http://github.com/calamares> ===
  *
  *   Copyright 2016, Teo Mrnjavac <teo@kde.org>
+ *   Copyright 2018, Adriaan de Groot <groot@kde.org>
  *
  *   Calamares is free software: you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -19,8 +20,9 @@
 #ifndef PYTHONQTGLOBALSTORAGEWRAPPER_H
 #define PYTHONQTGLOBALSTORAGEWRAPPER_H
 
-
 #include <QObject>
+#include <QStringList>
+#include <QVariant>
 
 namespace Calamares
 {