Install links for zpool feature compat aliases

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

Reviewed by: mm
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D28925

(cherry picked from commit 2ae79aa362e7a2ee72657b39be64f1390158aaf6)