dnstap io, fix compile warning when compiled with threading enabled.
@@ -1302,6 +1302,7 @@ int dt_io_thread_start(struct dt_io_thread* dtio, void* event_base_nothr)
dtio->started = 1;
#ifndef THREADS_DISABLED
ub_thread_create(&dtio->tid, dnstap_io, dtio);
+ (void)event_base_nothr;
#else
dtio->event_base = event_base_nothr;
dtio_setup_cmd(dtio);
