librt: Chase 315ee00fa961, fixing boot

libzfs uses librt as a dependency. Following 315ee00fa961 systems with
a separate / and /usr will fail to load the libzfs.so library because
librt.so is not available due to the fact that /usr is not mounted yet.
Install librt in /lib making it available to libzfs.

Reported by: emaste, imp
Fixes: 315ee00fa961
Differential Revision: https://reviews.freebsd.org/D41612