go.net/ipv4: add stubs for nacl build

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/95560048
diff --git a/ipv4/control_stub.go b/ipv4/control_stub.go
index 49b7f5c..f626de6 100644
--- a/ipv4/control_stub.go
+++ b/ipv4/control_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build plan9 solaris
+// +build nacl plan9 solaris
 
 package ipv4
 
diff --git a/ipv4/dgramopt_stub.go b/ipv4/dgramopt_stub.go
index 794048f..bee9e04 100644
--- a/ipv4/dgramopt_stub.go
+++ b/ipv4/dgramopt_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build plan9 solaris
+// +build nacl plan9 solaris
 
 package ipv4
 
diff --git a/ipv4/genericopt_stub.go b/ipv4/genericopt_stub.go
index 342dfcf..b202744 100644
--- a/ipv4/genericopt_stub.go
+++ b/ipv4/genericopt_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build plan9 solaris
+// +build nacl plan9 solaris
 
 package ipv4
 
diff --git a/ipv4/helper_stub.go b/ipv4/helper_stub.go
index 03432d3..314d4d5 100644
--- a/ipv4/helper_stub.go
+++ b/ipv4/helper_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build plan9 solaris
+// +build nacl plan9 solaris
 
 package ipv4
 
diff --git a/ipv4/sockopt_stub.go b/ipv4/sockopt_stub.go
index 53ae402..7bcc893 100644
--- a/ipv4/sockopt_stub.go
+++ b/ipv4/sockopt_stub.go
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build plan9 solaris
+// +build nacl plan9 solaris
 
 package ipv4