rpc: Fix compile error due to potentially unused parameter

connectDBus is only used if HAVE_DBUS is set. Therefore mark
it as potentially unused.

src/rpc/virnetserver.c

@@ -277,7 +277,7 @@ virNetServerPtr virNetServerNew(size_t min_workers,
                                 size_t max_workers,
                                 size_t max_clients,
                                 const char *mdnsGroupName,
-                                bool connectDBus,
+                                bool connectDBus ATTRIBUTE_UNUSED,
                                 virNetServerClientInitHook clientInitHook)
 {
     virNetServerPtr srv;