wgengine/netstack: use build tags to exclude gVisor GRO importation on iOS (#13015)

Updates tailscale/corp#22125

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/netstack/gro_default.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ios
+
+package netstack
+
+import (
+	nsgro "gvisor.dev/gvisor/pkg/tcpip/stack/gro"
+)
+
+// gro wraps a gVisor GRO implementation. It exists solely to prevent iOS from
+// importing said package (see _ios.go).
+type gro struct {
+	nsgro.GRO
+}