qt6_qml_type_registration() generates CMP0116 warnings with CMake 3.20

When using the Ninja generator, qt6_qml_type_registration() writes out a *.d depfile. CMake 3.20 introduced policy CMP0116 which now warns when this file is specified by or uses relative paths instead of absolute (see https://gitlab.kitware.com/cmake/cmake/-/issues/21959 for a related bug in CMake 3.20.0-rc5 that will hide this one until fixed). The name of the cpp file embedded in the *.d files is given as a path relative to the project's BINARY_DIR, but it needs to be an absolute path to give the correct, warning-free behavior.