src: use namespace prefix osmo_signal*

Summary of changes:

s/signal_cbfn/osmo_signal_cbfn/g
s/register_signal_handler/osmo_signal_register_handler/g
s/unregister_signal_handler/osmo_signal_unregister_handler/g
s/dispatch_signal/osmo_signal_dispatch/g
diff --git a/openbsc/src/osmo-bsc/osmo_bsc_audio.c b/openbsc/src/osmo-bsc/osmo_bsc_audio.c
index 515cfa7..64260aa 100644
--- a/openbsc/src/osmo-bsc/osmo_bsc_audio.c
+++ b/openbsc/src/osmo-bsc/osmo_bsc_audio.c
@@ -65,6 +65,6 @@
 int osmo_bsc_audio_init(struct gsm_network *net)
 {
 	net->hardcoded_rtp_payload = 98;
-	register_signal_handler(SS_ABISIP, handle_abisip_signal, net);
+	osmo_signal_register_handler(SS_ABISIP, handle_abisip_signal, net);
 	return 0;
 }