ipv4: add missing IP_HDRINCL option on Windows

Change-Id: Ibb7dacadef404f6fc4e6c744a5015835e580c97e
Reviewed-on: https://go-review.googlesource.com/34094
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv4/sys_windows.go

@@ -51,6 +51,7 @@ var (
 		ssoMulticastTTL:       {sysIP_MULTICAST_TTL, ssoTypeInt},
 		ssoMulticastInterface: {sysIP_MULTICAST_IF, ssoTypeInterface},
 		ssoMulticastLoopback:  {sysIP_MULTICAST_LOOP, ssoTypeInt},
+		ssoHeaderPrepend:      {sysIP_HDRINCL, ssoTypeInt},
 		ssoJoinGroup:          {sysIP_ADD_MEMBERSHIP, ssoTypeIPMreq},
 		ssoLeaveGroup:         {sysIP_DROP_MEMBERSHIP, ssoTypeIPMreq},
 	}