osmo-mgw.spec.in: Use %config(noreplace) to retain current config file

Change-Id: I37c130d9715a6826cc338f77edcd3cbec762fffd
diff --git a/contrib/osmo-mgw.spec.in b/contrib/osmo-mgw.spec.in
index 09d5a09..d24417e 100644
--- a/contrib/osmo-mgw.spec.in
+++ b/contrib/osmo-mgw.spec.in
@@ -114,7 +114,7 @@
 %{_bindir}/osmo-mgw
 %{_unitdir}/osmo-mgw.service
 %dir %{_sysconfdir}/osmocom
-%config %{_sysconfdir}/osmocom/osmo-mgw.cfg
+%config(noreplace) %{_sysconfdir}/osmocom/osmo-mgw.cfg
 
 %files -n libosmo-mgcp-client6
 %{_libdir}/libosmo-mgcp-client.so.6*