unix: add Linux lwtunnel constants for use with rtnetlink APIs

Change-Id: Ibe39b315321c18916870dcfc97b083a18b91be2b
Reviewed-on: https://go-review.googlesource.com/c/sys/+/271077
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/mkerrors.sh

@@ -225,6 +225,7 @@ struct ltchars {
 #include <linux/kexec.h>
 #include <linux/keyctl.h>
 #include <linux/loop.h>
+#include <linux/lwtunnel.h>
 #include <linux/magic.h>
 #include <linux/memfd.h>
 #include <linux/module.h>
@@ -561,6 +562,7 @@ ccflags="$@"
 		$2 ~ /^CRYPTO_/ ||
 		$2 ~ /^TIPC_/ ||
 		$2 ~ /^DEVLINK_/ ||
+		$2 ~ /^LWTUNNEL_IP/ ||
 		$2 !~ "WMESGLEN" &&
 		$2 ~ /^W[A-Z0-9]+$/ ||
 		$2 ~/^PPPIOC/ ||