This file is indexed.

/lib/modules/4.15.0-1021-gcp/build is in linux-headers-4.15.0-1021-gcp 4.15.0-1021.22~16.04.1.

This file is owned by root:root, with mode 0o777.

It is a symlink to /usr/src/linux-headers-4.15.0-1021-gcp