ns: Add logging support

Currently there is not support for Network Service (NS) logging.

This commit adds the missing definitions and sets the default level
to INFO. Further configuration can now be done with the 'logging
level ns' VTY command.

Sponsored-by: On-Waves ehf
diff --git a/src/pcu_main.cpp b/src/pcu_main.cpp
index 0cbced8..ed51212 100644
--- a/src/pcu_main.cpp
+++ b/src/pcu_main.cpp
@@ -194,6 +194,7 @@
 	msgb_set_talloc_ctx(tall_pcu_ctx);
 
 	osmo_init_logging(&gprs_log_info);
+	gprs_ns_set_log_ss(DNS);
 	bssgp_set_log_ss(DBSSGP);
 
 	vty_init(&pcu_vty_info);