internal/nettest: fix build on nacl

Change-Id: Ib3cd96ec4156737d80feda0504c349da5d41dc75
Reviewed-on: https://go-review.googlesource.com/11603
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/nettest/rlimit_plan9.go → internal/nettest/rlimit_stub.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 nacl plan9
+
 package nettest
 
 func maxOpenFiles() int { return defaultMaxOpenFiles }