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/sgsn/osmo-sgsn.cfg b/sgsn/osmo-sgsn.cfg
index 811d3ff..dfbff6c 100644
--- a/sgsn/osmo-sgsn.cfg
+++ b/sgsn/osmo-sgsn.cfg
@@ -21,6 +21,7 @@
  logging level ref notice
  logging level gprs debug
  logging level lns debug
+ logging level lnssignal debug
  logging level lbssgp info
  logging level llc debug
  logging level sndcp debug