cbsp: Add osmo_cbsp_segmentation_cb for message segmentation

This call-back can for example be used as segmentation call-back
for libosmo-netif stream_cli/stream_srv or directly for osmo_io.

Related: OS#5755
Change-Id: I5e922c54b3431d759b38e81e55076125c5a34008
diff --git a/src/gsm/libosmogsm.map b/src/gsm/libosmogsm.map
index db2dbcb..2c4c621 100644
--- a/src/gsm/libosmogsm.map
+++ b/src/gsm/libosmogsm.map
@@ -793,6 +793,7 @@
 osmo_cbsp_decode;
 osmo_cbsp_recv_buffered;
 osmo_cbsp_errstr;
+osmo_cbsp_segmentation_cb;
 
 osmo_i460_demux_in;
 osmo_i460_mux_enqueue;