Doesn't build without -DENABLE_QT=ON

[alyssais]

Even with qtbase available, unless I explicitly set -DENABLE_QT=ON, I get compiler errors like this:

In file included from /build/source/src/actions/../mmg-utils.h:20,
                 from /build/source/src/actions/../mmg-message.h:22,
                 from /build/source/src/actions/mmg-action.h:22,
                 from /build/source/src/actions/mmg-action-video-sources.h:20,
                 from /build/source/src/actions/mmg-action-video-sources.cpp:19:
/build/source/src/actions/../obs-midi-mg.h:22:10: fatal error: QObject: No such file or directory
   22 | #include <QObject>
      |          ^~~~~~~~~

If some QT things are always needed, even if -DENABLE_QT=OFF, there probably needs to be an unconditional find_package for Qt6.