/lib/modules/3.13.0-51-generic/build is in linux-headers-3.13.0-51-generic 3.13.0-51.84.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-3.13.0-51-generic
/usr/src/linux-headers-3.13.0-51-generic