wgengine/netstack/gro: permit building without GRO

This only saves ~32KB in the minimal linux/amd64 binary, but it's a
step towards permitting not depending on gvisor for small builds.

Updates #17283

Change-Id: Iae8da5e9465127de354dbcaf25e794a6832d891b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/netstack/gro/gro_default.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !ios
+//go:build !ios && !ts_omit_gro
 
 package gro
 

wgengine/netstack/gro/gro_disabled.go

@@ -1,22 +1,27 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build ios
+//go:build ios || ts_omit_gro
 
 package gro
 
 import (
-	"gvisor.dev/gvisor/pkg/tcpip/stack"
+	"runtime"
+
 	"tailscale.com/net/packet"
 )
 
 type GRO struct{}
 
 func NewGRO() *GRO {
-	panic("unsupported on iOS")
+	if runtime.GOOS == "ios" {
+		panic("unsupported on iOS")
+	}
+	panic("GRO disabled in build")
+
 }
 
-func (g *GRO) SetDispatcher(_ stack.NetworkDispatcher) {}
+func (g *GRO) SetDispatcher(any) {}
 
 func (g *GRO) Enqueue(_ *packet.Parsed) {}