zfs: Fix debug mode build

The undefined reference error only shows up when building OSv on debug
mode (mode=debug):

  bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.o: In function `zfs_setattr':
  bsd/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/zfs_vnops.c:2529: undefined reference to `zfs_xvattr_set

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>