Add const to nvlist functions to properly expose their real behavior

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #12728

lib/libzutil/zutil_import.c

@@ -749,7 +749,7 @@ get_configs(libpc_handle_t *hdl, pool_list_t *pl, boolean_t active_ok,
 		    nvlist_add_uint64(nvroot, ZPOOL_CONFIG_ID, 0ULL) != 0 ||
 		    nvlist_add_uint64(nvroot, ZPOOL_CONFIG_GUID, guid) != 0 ||
 		    nvlist_add_nvlist_array(nvroot, ZPOOL_CONFIG_CHILDREN,
-		    child, children) != 0) {
+		    (const nvlist_t **)child, children) != 0) {
 			nvlist_free(nvroot);
 			goto nomem;
 		}