doc/examples/osmo-ggsn-kernel-gtp.cfg: new file

Add a copy of osmo-ggsn.cfg, with gtpu-mode set to kernel-gtp and apn
inet6 and inet46 removed (as the kernel module only supports ipv4).

Related: OS#3209
Change-Id: I6fbe8a8e55bad41532e9aed3cf71ebebffdcee52
diff --git a/contrib/osmo-ggsn.spec.in b/contrib/osmo-ggsn.spec.in
index 0a568ef..6d55af8 100644
--- a/contrib/osmo-ggsn.spec.in
+++ b/contrib/osmo-ggsn.spec.in
@@ -108,6 +108,7 @@
 %{_mandir}/man8/sgsnemu.8%{?ext_man}
 %{_unitdir}/%{name}.service
 %dir %{_docdir}/%{name}/examples
+%{_docdir}/%{name}/examples/osmo-ggsn-kernel-gtp.cfg
 %{_docdir}/%{name}/examples/osmo-ggsn.cfg
 %{_docdir}/%{name}/examples/sgsnemu.conf
 %dir %{_sysconfdir}/osmocom