add consistent SPDX copyright headers

lib-sql/tables.sql

@@ -1,3 +1,10 @@
+-- SPDX-License-Identifier: GPL-2.0-only
+--
+-- This file is part of Nominatim. (https://nominatim.org)
+--
+-- Copyright (C) 2022 by the Nominatim developer community.
+-- For a full list of authors see the git log.
+
 drop table if exists import_status;
 CREATE TABLE import_status (
   lastimportdate timestamp with time zone NOT NULL,