/usr/src/linux-headers-4.13.0-41-generic/Kbuild is in linux-headers-4.13.0-41-generic 4.13.0-41.46~16.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.13.0-41-generic/../linux-headers-4.13.0-41/Kbuild