internal/netreflect: fix test on NaCl and Plan 9

Change-Id: Ia8bc87d0398951d7be74779279dca177f499d293
Reviewed-on: https://go-review.googlesource.com/36377
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

internal/netreflect/socket_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
+
 package netreflect_test
 
 import (