osmo-mgw: Update copyright statement

sysmocom (specifically Philipp) was doing all the new osmo-mgw
development, but that is not yet reflected in the Copyright statement
for some reason.  Let's fix it.

Change-Id: I4cad29daaabec1caec1bd09088414e59fa15a17e
diff --git a/src/osmo-mgw/mgw_main.c b/src/osmo-mgw/mgw_main.c
index 2e1ac8b..e21baa8 100644
--- a/src/osmo-mgw/mgw_main.c
+++ b/src/osmo-mgw/mgw_main.c
@@ -4,6 +4,7 @@
 /*
  * (C) 2009-2011 by Holger Hans Peter Freyther <zecke@selfish.org>
  * (C) 2009-2011 by On-Waves
+ * (C) 2017 by sysmocom - s.f.m.c. GmbH, Author: Philipp Maier
  * All Rights Reserved
  *
  * This program is free software; you can redistribute it and/or modify