mgw: Test IPv6 addresses in MGCP messages

These tests verify that osmo-mgw handles requests no matter of IP
version used in the remote address.

Change-Id: Ib134dda5a8f7c8f50968b6ce330f9986ae72575d
diff --git a/mgw/osmo-mgw.cfg b/mgw/osmo-mgw.cfg
index edc8190..beb6fdf 100644
--- a/mgw/osmo-mgw.cfg
+++ b/mgw/osmo-mgw.cfg
@@ -22,6 +22,7 @@
   bind port 2427
   rtp port-range 4002 16000
   rtp bind-ip 127.0.0.1
+  rtp bind-ip-v6 ::1
   rtp ip-probing
   rtp ip-dscp 184
   no rtp keep-alive