gsm0808: Add value strings for BSSMAP cause codes

libosmocore has no value strings for BSSMAP cause codes yet.

- Add value strings for BSSMAP cause codes and a function
  to retrieve them

Change-Id: I313dd8d7b06374e1e35ddc18b7a42562d9e25d45
Related: OS#1609
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index fd07704..dd4722e 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -139,6 +139,7 @@
 gsm0808_att_tlvdef;
 gsm0808_bssap_name;
 gsm0808_bssmap_name;
+gsm0808_cause_name;
 gsm0808_create_ass;
 gsm0808_create_assignment_completed;
 gsm0808_create_ass_compl;