Add function gprs_nsvc_state_append

A common function to append the nsvc state from osmo-sgsn or osmo-gbproxy

Change-Id: I7f0eaff7329ab98cad792d30b20ab053007aab85
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 83a3621..d56e651 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -70,6 +70,7 @@
 gprs_nsvc_reset;
 gprs_nsvc_by_nsvci;
 gprs_nsvc_by_nsei;
+gprs_nsvc_state_append;
 
 gprs_log_filter_fn;