ipn/ipnlocal: add webclient support for tvOS (#20256)

updates tailscale/corp#44019

WebClient is very useful for remote management
on tvOS (which cannot do ssh).   Let's include it there.
Minimal corresponding tailscale/corp changes to follow
to add UI to set the required prefs.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

ipn/ipnlocal/web_client.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !ios && !android && !ts_omit_webclient
+//go:build !android && !ts_omit_webclient
 
 package ipnlocal
 

ipn/ipnlocal/web_client_stub.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build ios || android || ts_omit_webclient
+//go:build android || ts_omit_webclient
 
 package ipnlocal