net/tstun: don't compile the code New constructor on js/wasm

Updates #3157

Change-Id: I81603edf3e69e6f1517b0074eef6b648f2981c50
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/tstun/tun.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 tun creates a tuntap device, working around OS-specific
 // quirks if necessary.
 package tstun