ssh/tailssh: add Plan 9 support for Tailscale SSH

Updates #5794

Change-Id: I7b05cd29ec02085cb503bbcd0beb61bf455002ac
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/ssh.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build (linux || darwin || freebsd || openbsd) && !ts_omit_ssh
+//go:build (linux || darwin || freebsd || openbsd || plan9) && !ts_omit_ssh
 
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