ns/gbproxy/sgsn: Update configs to use logging level lnssignal'

In libosmocore I34b8fde2955ecc010d1dcd9512e1bba9211e2c0d we introduced
a new log subsystem; enabel it in the related configs here

Change-Id: Ie3d178e68aa81d5636c87940074cb6582ac2f131
diff --git a/gbproxy/osmo-gbproxy.cfg b/gbproxy/osmo-gbproxy.cfg
index b2ccb3b..dab4ef3 100644
--- a/gbproxy/osmo-gbproxy.cfg
+++ b/gbproxy/osmo-gbproxy.cfg
@@ -10,6 +10,7 @@
  logging timestamp 1
  logging print file 1
  logging level lns debug
+ logging level lnssignal debug
  logging level lbssgp debug
  logging level gprs debug
 line vty