LU-10536 build: add -lnvpair to ZFS LDFLAGS

LU-10536 build: add -lnvpair to ZFS LDFLAGS

The ZFS mount utils plugin directly depends on libnvpair so add
-lnvpair to the relevant LDFLAGS. This makes building with
--disable-shared succeed.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ie7d7043b011dab3e139786ed8fffb09e1078a34b
Reviewed-on: https://review.whamcloud.com/30947
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>