hnbap_common: Add function to HNBAP generate initiating message

An earlier version was commented out, this function now works like
hnbap_generate_successful_outcome()
diff --git a/src/hnbap_common.h b/src/hnbap_common.h
index 17760fe..8726718 100644
--- a/src/hnbap_common.h
+++ b/src/hnbap_common.h
@@ -135,6 +135,11 @@
 
 extern int asn1_xer_print;
 
+struct msgb *hnbap_generate_initiating_message(
+					 e_ProcedureCode procedureCode,
+					 Criticality_t criticality,
+					 asn_TYPE_descriptor_t * td, void *sptr);
+
 struct msgb *hnbap_generate_successful_outcome(
 					   e_ProcedureCode procedureCode,
 					   Criticality_t criticality,