cmd/tsshd: empty file for windows build

Signed-off-by: David Crawshaw <david@zentus.com>

cmd/tsshd/tsshd_windows.go

@@ -0,0 +1,11 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+
+package main
+
+func main() {
+	panic("tsshd does not work on windows yet")
+}