go.net/ipv4: fix build with go1.2

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/162110043

ipv4/zsys_dragonfly.go

diff --git a/ipv4/zsys_dragonfly.go b/ipv4/zsys_dragonfly.go
index 2c15bf4..f5c9cce 100644
--- a/ipv4/zsys_dragonfly.go
+++ b/ipv4/zsys_dragonfly.go
@@ -1,6 +1,8 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs defs_dragonfly.go
 
+// +build dragonfly
+
 package ipv4
 
 const (

ipv4/zsys_solaris.go

diff --git a/ipv4/zsys_solaris.go b/ipv4/zsys_solaris.go
index 52b6dd6..d7c2334 100644
--- a/ipv4/zsys_solaris.go
+++ b/ipv4/zsys_solaris.go
@@ -1,7 +1,7 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs defs_solaris.go
 
-// +build go1.3
+// +build solaris
 
 package ipv4