04.80: New gsm0480_gen_{reject,return_error}() functions

Add functions to generate TS 04.80 (supplementary services)
Reject and ReturnError components.

Change-Id: I6e5ee39c3d03364f7833ec717593d5ddb0a4c5f9
diff --git a/include/osmocom/gsm/gsm0480.h b/include/osmocom/gsm/gsm0480.h
index 9fa84b0..246f4b3 100644
--- a/include/osmocom/gsm/gsm0480.h
+++ b/include/osmocom/gsm/gsm0480.h
@@ -110,6 +110,8 @@
 
 struct msgb *gsm0480_msgb_alloc_name(const char *name);
 struct msgb *gsm0480_gen_ussd_resp_7bit(uint8_t invoke_id, const char *text);
+struct msgb *gsm0480_gen_return_error(uint8_t invoke_id, uint8_t error_code);
+struct msgb *gsm0480_gen_reject(int invoke_id, uint8_t problem_tag, uint8_t problem_code);
 
 struct msgb *gsm0480_create_ussd_resp(uint8_t invoke_id, uint8_t trans_id, const char *text);
 struct msgb *gsm0480_create_unstructuredSS_Notify(int alertPattern, const char *text);