oap_client: Fix license: GPLv2+ instead of AGPLv3+

libosmo{core,gsm,vty} code is GPLv2+.  The OAP code originated in
osmo-msc.git and was moved here without changing the license. That
was a mistake, it always was meant to be under GPLv2-or-later after
moving to libosmocore.git.

Copyright is with sysmocom, so I as the managing director can
approve the license change.

Change-Id: I08311fa8214c15f8df8945b9894226608cf96f15
diff --git a/tests/oap/oap_client_test.c b/tests/oap/oap_client_test.c
index a841b38..dadf2ee 100644
--- a/tests/oap/oap_client_test.c
+++ b/tests/oap/oap_client_test.c
@@ -4,18 +4,19 @@
  * All Rights Reserved
  *
  * This program is free software; you can redistribute it and/or modify
- * it under the terms of the GNU Affero General Public License as published by
- * the Free Software Foundation; either version 3 of the License, or
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
  * (at your option) any later version.
  *
  * This program is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU Affero General Public License for more details.
+ * GNU General Public License for more details.
  *
- * You should have received a copy of the GNU Affero General Public License
+ * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *
+ * SPDX-License-Identifier: GPLv2+
  */
 
 #include <osmocom/core/application.h>