Drop third parameter from FC_QT6_IF_QT6_MOC
What seems like autoconf bug, third parameter of FC_QT6_IF_QT6_MOC is causing problems on older systems with #43331. As there is no users for that parameter (which might be part of the problem - it's always empty), just drop it.
What seems like autoconf bug, third parameter of FC_QT6_IF_QT6_MOC is causing problems on older systems with #43331. As there is no users for that parameter (which might be part of the problem - it's always empty), just drop it.