gsm48_rest_octets: Add parser for SI4 rest octets

Introduces osmo_gsm48_rest_octets_si4_decode()

Change-Id: I9d6ed06731ae15fdcef1a1f397d6ac2b7b1ca980
Related: OS#3075
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index dd28a15..031cc20 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -322,6 +322,7 @@
 osmo_gsm48_rest_octets_si4_encode;
 osmo_gsm48_rest_octets_si13_encode;
 osmo_gsm48_rest_octets_si3_decode;
+osmo_gsm48_rest_octets_si4_decode;
 gsm48_rr_msg_name;
 gsm48_cc_state_name;
 gsm48_construct_ra;