This matches the other options that govern Qt dependencies. My motivation for this change is that I got a build error in KScreenLocker because I had built KConfig before QtDeclarative:
CMake Error at CMakeLists.txt:192 (add_library): Target "KScreenLocker" links to target "KF5::ConfigQml" but the target was not found. Perhaps a find_package() call is missing for an IMPORTED target, or an ALIAS target is missing
With this change, KF5::ConfigQml will always be available unless the user has explicitly asked for KConfig to be built without Qml support rather than implicitly making that decision based on the presence of QtQml.