all: adjust some build tags for plan9

I'm not saying it works, but it compiles.

Updates #5794

Change-Id: I2f3c99732e67fe57a05edb25b758d083417f083e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tstun_plan9.go

@@ -0,0 +1,17 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tstun
+
+import (
+	"github.com/tailscale/wireguard-go/tun"
+	"tailscale.com/types/logger"
+)
+
+func New(logf logger.Logf, tunName string) (tun.Device, string, error) {
+	panic("not implemented")
+}
+
+func Diagnose(logf logger.Logf, tunName string, err error) {
+	panic("not implemented")
+}

net/tstun/tun.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !wasm
+//go:build !wasm && !plan9
 
 // Package tun creates a tuntap device, working around OS-specific
 // quirks if necessary.