ipn/ipnserver: fix build on js/wasm

Broken by 3dedcd1640 but we don't have CI coverage yet.

Updates #3157

Change-Id: Ie8e95ebd36264887fdeed16fc9f25a857d48124b
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/proxyconnect.go

@@ -2,6 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+//go:build !js
+// +build !js
+
 package ipnserver
 
 import (

ipn/ipnserver/proxyconnect_js.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2022 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.
+
+package ipnserver
+
+import (
+	"bufio"
+	"context"
+	"net"
+
+	"tailscale.com/types/logger"
+)
+
+func (s *Server) handleProxyConnectConn(ctx context.Context, br *bufio.Reader, c net.Conn, logf logger.Logf) {
+	panic("unreachable")
+}