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/bts_siemens_bs11.c b/openbsc/src/libbsc/bts_siemens_bs11.c
index 57625cd..44f9358 100644
--- a/openbsc/src/libbsc/bts_siemens_bs11.c
+++ b/openbsc/src/libbsc/bts_siemens_bs11.c
@@ -583,8 +583,8 @@
 	gsm_btsmodel_set_feature(&model_bs11, BTS_FEAT_HOPPING);
 	gsm_btsmodel_set_feature(&model_bs11, BTS_FEAT_HSCSD);
 
-	register_signal_handler(SS_INPUT, inp_sig_cb, NULL);
-	register_signal_handler(SS_GLOBAL, gbl_sig_cb, NULL);
+	osmo_signal_register_handler(SS_INPUT, inp_sig_cb, NULL);
+	osmo_signal_register_handler(SS_GLOBAL, gbl_sig_cb, NULL);
 
 	return gsm_bts_model_register(&model_bs11);
 }