Install links for zpool feature compat aliases

[freqlabs]

Summary

The alias links were missed when this feature was introduced to the FreeBSD build system in 10f57cb98fd61b2669640a84aa73ad118601f281.

Test Plan

$ ls /usr/share/zfs/compatability.d
2018                compat-2018         freebsd-11.0        freebsd-11.4        freenas-11.0        freenas-9.10.2      openzfsonosx-1.7.0  truenas-12.0        zol-0.7
2019                compat-2019         freebsd-11.1        freebsd-12.0        freenas-11.1        grub2               openzfsonosx-1.8.1  ubuntu-18.04        zol-0.8
2020                compat-2020         freebsd-11.2        freebsd-12.1        freenas-11.2        openzfs-2.0-freebsd openzfsonosx-1.9.3  ubuntu-20.04
2021                compat-2021         freebsd-11.3        freebsd-12.2        freenas-11.3        openzfs-2.0-linux   openzfsonosx-1.9.4  zol-0.6.5