libgb: make sure all BSSGP functions have bssgp_ prefix

We change the minority of functions employing the gprs_bssgp_ prefix to
match with the majority without gprs_ in front.
diff --git a/openbsc/src/gprs/gb_proxy_main.c b/openbsc/src/gprs/gb_proxy_main.c
index 00a134f..b5b11fd 100644
--- a/openbsc/src/gprs/gb_proxy_main.c
+++ b/openbsc/src/gprs/gb_proxy_main.c
@@ -262,7 +262,7 @@
 	gbcfg.nsi = bssgp_nsi;
 	gprs_ns_vty_init(bssgp_nsi);
 	gprs_ns_set_log_ss(DNS);
-	gprs_bssgp_set_log_ss(DBSSGP);
+	bssgp_set_log_ss(DBSSGP);
 	osmo_signal_register_handler(SS_L_NS, &gbprox_signal, NULL);
 
 	rc = gbproxy_parse_config(config_file, &gbcfg);