qt: Add #include <QStringList> needed for building against qt6
* lang/qt/src/downloadjob.h, lang/qt/src/exportjob.h, lang/qt/src/keylistjob.h lang/qt/src/refreshkeysjob.h: in qt6 we need to use #include <QStringList>
It fixes some compile qt6 compile error.
- Signed-off-by: Laurent Montel <montel@kde.org>