net: fix windows build

R=golang-dev, r2
CC=golang-dev
https://golang.org/cl/4184051
diff --git a/src/pkg/net/multicast_test.go b/src/pkg/net/multicast_test.go
index 0c9a035..06d430e 100644
--- a/src/pkg/net/multicast_test.go
+++ b/src/pkg/net/multicast_test.go
@@ -5,10 +5,14 @@
 package net
 
 import (
+	"runtime"
 	"testing"
 )
 
 func TestMulticastJoinAndLeave(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		return
+	}
 	addr := &UDPAddr{
 		IP:   IPv4zero,
 		Port: 0,