Implement ITU-T I.460 multiplex / demultiplex

This implements a multiplexer and de-multiplexer for the ITU-T I.460
standard.  The latter covers the transmission of sub-slots of 32/16/8k
inside 64k timeslots.

Change-Id: Id522f06e73b77332b437b7a27e4966872da70eda
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index 70b3916..2000e6c 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -667,5 +667,12 @@
 osmo_cbsp_recv_buffered;
 osmo_cbsp_errstr;
 
+osmo_i460_demux_in;
+osmo_i460_mux_enqueue;
+osmo_i460_mux_out;
+osmo_i460_subchan_add;
+osmo_i460_subchan_del;
+osmo_i460_ts_init;
+
 local: *;
 };