Fix CMake config file for Qt6 with X11 builds
In that case Qt5X11Extras_FOUND is undefined and would expand to an empty string here, resulting in invalid CMake code. Instead, let's check for the Qt6 target, that doesn't need variable expansion.
In that case Qt5X11Extras_FOUND is undefined and would expand to an empty string here, resulting in invalid CMake code. Instead, let's check for the Qt6 target, that doesn't need variable expansion.