serial: json: update b_dict usage

serial/json.c

@@ -2978,7 +2978,7 @@ static enum b_status serialise_dict(const struct b_object *object, cJSON **out)
 			return status;
 		}
 
-		cJSON_AddItemToObject(json_dict, it.key, child);
+		cJSON_AddItemToObject(json_dict, b_string_ptr(it.key), child);
 	}
 
 	*out = json_dict;