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/libbsc/handover_decision.c b/openbsc/src/libbsc/handover_decision.c
index d6fd4f4..24c0f79 100644
--- a/openbsc/src/libbsc/handover_decision.c
+++ b/openbsc/src/libbsc/handover_decision.c
@@ -293,5 +293,5 @@
 
 void on_dso_load_ho_dec(void)
 {
-	register_signal_handler(SS_LCHAN, ho_dec_sig_cb, NULL);
+	osmo_signal_register_handler(SS_LCHAN, ho_dec_sig_cb, NULL);
 }