reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf.git/commit/?id=20d564bba6b3806c26498061299a88330561efa5
        git remote add netfilter-nf https://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf.git
        git fetch --no-tags netfilter-nf testing
        git checkout 20d564bba6b3806c26498061299a88330561efa5
        # apt-get install llvm-dev

        # save the config file
        make W=1 ARCH=i386 olddefconfig
        make W=1 ARCH=i386 
        make W=1 headers_install
        make W=1 -C tools/testing/selftests