qt_add_dbus_adaptor and qt_add_dbus_interface regression

I'm working on a fix so you can assign this issue to me.

Recent changes to qdbusxml2cpp break qt_add_dbus_adaptor and qt_add_dbus_interface.

The problem is in Qt6DBusMacros.cmake.

The 5th parameter for qt6_add_dbus_adaptor and the third parameter for qt_add_dbus_interface don't handle possible filename with relative path.

Example:

If the project-included CMakeLists.txt is in a subfolder called src/ that contains all the qt6_add_dbus_adaptor / qt_add_dbus_interface, and the actual interface header and xml's are in subdirs under src/, the following:

qt_add_dbus_adaptor(SOURCES dbus/org.mpris.MediaPlayer2.xml core/mpris2.h mpris::Mpris2 core/mpris2_root Mpris2Root)

Results in build_directory/src/moc_core/mpris2_root.cpp and the build fails.