cmd/tailscaled, net/tstun: build for aix/ppc64

At least in userspace-networking mode.

Fixes #11361

Change-Id: I78d33f0f7e05fe9e9ee95b97c99b593f8fe498f2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tstun_stub.go

@@ -1,6 +1,8 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
+//go:build plan9 || aix
+
 package tstun
 
 import (

net/tstun/tun.go

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