net/tstun: use ts_omit_gro in another place I missed earlier

I didn't notice this GRO code during b3ae1cb0cc.

Updates #17283

Change-Id: I95c06c19e489097fc8d61180dc57ae4b8a69c58c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/wrap_linux.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build linux && !ts_omit_gro
+
 package tstun
 
 import (