another termination fix

espeak_thread needs to signal softsynth_thread once more as it is going
town so that softsynth_thread will see that should_run is now 0 and
terminate.

synth.c

@@ -277,6 +277,7 @@ void *espeak_thread(void *arg)
 			pthread_cond_signal(&stop_acknowledged);
 		}
 	}
+	pthread_cond_signal(&stop_acknowledged);
 	pthread_mutex_unlock(&queue_guard);
 	return NULL;
 }