This is about Python bindings, clang is needed as a build dependency for them.
distro-dependencies/debian.ini · master · Sysadmin / Git Repository Metadata · GitLab needs to be updated to include them. You could even attempt to send an MR there if you want. In the meantime, just installing clang (maybe clang-dev?) should work, as you found out. You didn’t do anything wrong.
You can also just not build the Python bindings, that’s fine too. That can be done by setting -DBUILD_PYTHON_BINDINGS=OFF in the global cmake-options in ~/.config/kde-builder.yaml.