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

LGTM=minux
R=golang-codereviews, minux
CC=golang-codereviews
https://golang.org/cl/146890043

ipv4/zsys_solaris.go

@@ -1,6 +1,8 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs defs_solaris.go
 
+// +build go1.3
+
 package ipv4
 
 const (