Fix the FreeBSD userspace build (#15716)

- Mark some parameters to zpool_power*() as unused.
- Add a stub zpool_disk_wait().

Fixes: a9520e6e5 ("zpool: Add slot power control, print power status")

Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

lib/libzutil/os/freebsd/zutil_import_os.c

@@ -263,3 +263,11 @@ update_vdevs_config_dev_sysfs_path(nvlist_t *config)
 {
 	(void) config;
 }
+
+int
+zpool_disk_wait(const char *path)
+{
+
+	(void) path;
+	return (ENOTSUP);
+}