wgengine/netstack/gro: exclude importation of gVisor GRO pkg on iOS (#13202)

In df6014f1d7 we removed build tag
gating preventing importation, which tripped a NetworkExtension limit
test in corp. This was a reversal of
25f0a3fc8f which actually made the
situation worse, hence the simplification.

This commit goes back to the strategy in
25f0a3fc8f, and gets us back under the
limit in my local testing. Admittedly, we don't fully understand
the effects of importing or excluding importation of this package,
and have seen mixed results, but this commit allows us to move forward
again.

Updates tailscale/corp#22125

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

wgengine/netstack/gro/gro_ios.go

@@ -0,0 +1,23 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build ios
+
+package gro
+
+import (
+	"gvisor.dev/gvisor/pkg/tcpip/stack"
+	"tailscale.com/net/packet"
+)
+
+type GRO struct{}
+
+func NewGRO() *GRO {
+	panic("unsupported on iOS")
+}
+
+func (g *GRO) SetDispatcher(_ stack.NetworkDispatcher) {}
+
+func (g *GRO) Enqueue(_ *packet.Parsed) {}
+
+func (g *GRO) Flush() {}