Explicitly link libzfs against libavl as it is done in OpenSolaris
(4543:12bb2876a62e). Without this, some third party applications
may break because the lack of AVL related symbols.

FreeBSD base system are not affected because the FreeBSD ZFS command
line tools were all linked against libavl and thus hide the underlying
issue.

PR: java/183081
Tested by: jkim
MFC after: 3 days