gprs: add value strings for NS PDU type

Add value strings for Service Control PDUs according to 3GPP TS 48.016 ยง9
and use them for logging.

Change-Id: I0ea3a45f35d68619d4cfa9735ef77abd9f9f0d58
Related: SYS#3610
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 9aec280..3ec3ee5 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -38,6 +38,7 @@
 bssgp_vty_init;
 bssgp_nsi;
 
+gprs_ns_pdu_strings;
 gprs_ns_cause_str;
 gprs_ns_destroy;
 gprs_ns_close;