Add libnvpair to mount_zfs dependencies

Commit e6f290535c2fff8b940c4eacb1c90cbc2cfc7508 added libzpool to
the mount_zfs dependencies. This brought in the nvpair symbols
which are used by libzpool. To resolve this include the libnvpair
library for mount_zfs even though mount_zfs doesn't directly
require any of these symbols.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #926