hswg: use a buffered channel for signals

Found by go vet.

hswg/main.go

@@ -494,7 +494,7 @@ func main() {
 		log.Fatalln(err)
 	}
 
-	signals := make(chan os.Signal)
+	signals := make(chan os.Signal, 1)
 	signal.Notify(signals, syscall.SIGINT, syscall.SIGHUP, syscall.SIGTERM)
 	for {
 		select {