USSD: Introduce gsm0480_gen_ussd_resp_7bit()

Contrary to the existing gsm0480_create_ussd_resp(), the new function
only generates the value part of the FACILITY IE, and not the IE
Tag/Length or the 04.08 L3 header.  This is needed in the context of
GSUP-encapsulated USSD, as here we don't work with L3 messages, but
only pass on the FACILITY IE value.

Change-Id: Ide240279240322f643e142229eb7829f538c6314
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 312c990..d21514c 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -98,6 +98,7 @@
 gsm0480_comp_type_names;
 gsm0480_op_code_names;
 gsm0480_msgb_alloc_name;
+gsm0480_gen_ussd_resp_7bit;
 
 gsm0502_calc_paging_group;