go.net/ipv6: fix build with go1.2

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/163120043

ipv6/zsys_dragonfly.go

@@ -1,6 +1,8 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs defs_dragonfly.go
 
+// +build dragonfly
+
 package ipv6
 
 const (

ipv6/zsys_solaris.go

@@ -1,6 +1,8 @@
 // Created by cgo -godefs - DO NOT EDIT
 // cgo -godefs defs_solaris.go
 
+// +build solaris
+
 package ipv6
 
 const (