wake up espeak_thread when softsynth_thread terminates

espeak_thread needs a signal since it might be sleeping and
should_run has changed.  This makes sure it terminates.

softsynth.c

@@ -256,6 +256,7 @@ void *softsynth_thread(void *arg)
 		process_buffer(s, buf, length);
 		pthread_mutex_lock(&queue_guard);
 	}
+	pthread_cond_signal(&runner_awake);
 	pthread_mutex_unlock(&queue_guard);
 	return NULL;
 }