net: fix build with CGO_ENABLED=0

R=golang-dev, rsc, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/7857044

src/pkg/net/empty.c

+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is required to prevent compiler errors
+// when the package built with CGO_ENABLED=0.
+// Otherwise the compiler says:
+// pkg/net/fd_poll_runtime.go:15: missing function body