/lib/modules/4.4.0-138-lowlatency/build is in linux-headers-4.4.0-138-lowlatency 4.4.0-138.164~14.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.4.0-138-lowlatency
/usr/src/linux-headers-4.4.0-138-lowlatency