gb: Add bssgp_pdu_str to libosmogb namespace

[hfreyther: To be used by osmo-pcu]
diff --git a/src/gb/libosmogb.map b/src/gb/libosmogb.map
index 75406c0..9aec280 100644
--- a/src/gb/libosmogb.map
+++ b/src/gb/libosmogb.map
@@ -2,6 +2,7 @@
 global:
 bssgp_cause_str;
 bssgp_create_cell_id;
+bssgp_pdu_str;
 bssgp_fc_in;
 bssgp_fc_init;
 bssgp_fc_ms_init;