derp/derphttp: don't link websockets other than on GOOS=js

Or unless the new "ts_debug_websockets" build tag is set.

Updates #1278

Change-Id: Ic4c4f81c1924250efd025b055585faec37a5491d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/websocket.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build linux || js
+//go:build js || ((linux || darwin) && ts_debug_websockets)
 
 package derphttp
 
@@ -14,6 +14,8 @@ import (
 	"tailscale.com/net/wsconn"
 )
 
+const canWebsockets = true
+
 func init() {
 	dialWebsocketFunc = dialWebsocket
 }