Generate SI2bis Rest Octets

According to the spec it's an empty 1-byte element reserved for future
extension but we still have to generate padding properly. Add stub
function similar to the used for SI2ter and adjust test output
accordingly.

Related: OS#2711
Change-Id: I3c278c57880a173df3c4648c9724339d23ce94fd
diff --git a/include/osmocom/bsc/rest_octets.h b/include/osmocom/bsc/rest_octets.h
index ba25b6d..f7ad682 100644
--- a/include/osmocom/bsc/rest_octets.h
+++ b/include/osmocom/bsc/rest_octets.h
@@ -10,6 +10,7 @@
 int rest_octets_si1(uint8_t *data, uint8_t *nch_pos, int is1800_net);
 int rest_octets_si2quater(uint8_t *data, struct gsm_bts *bts);
 int rest_octets_si2ter(uint8_t *data);
+int rest_octets_si2bis(uint8_t *data);
 int rest_octets_si6(uint8_t *data, bool is1800_net);
 
 struct gsm48_si_selection_params {