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/debian/osmo-ggsn.examples b/debian/osmo-ggsn.examples
index fe0136c..f9e13b1 100644
--- a/debian/osmo-ggsn.examples
+++ b/debian/osmo-ggsn.examples
@@ -1,2 +1,3 @@
 doc/examples/osmo-ggsn.cfg
+doc/examples/osmo-ggsn-kernel-gtp.cfg
 doc/examples/sgsnemu.conf