Fix plugin query
QT_MAJOR_VERSION is undefined, the proper thing is QT_VERSION_MAJOR
This causes the plugin query to not find any plugins
Since we only target KF6 we might as well hardcode that
QT_MAJOR_VERSION is undefined, the proper thing is QT_VERSION_MAJOR
This causes the plugin query to not find any plugins
Since we only target KF6 we might as well hardcode that