/usr/src/linux-gcp-headers-4.18.0-1005/spl/rpm/Makefile.am is in linux-gcp-headers-4.18.0-1005 4.18.0-1005.6~18.04.1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1
SUBDIRS = generic redhat