/lib/modules/4.8.0-56-generic/build is in linux-headers-4.8.0-56-generic 4.8.0-56.61~16.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.8.0-56-generic
/usr/src/linux-headers-4.8.0-56-generic