net/dns: don't build init*.go on non-windows

To remove the regexp dep on iOS, notably.

Updates tailscale/corp#2238

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

net/dns/ini.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
+
 package dns
 
 import (

net/dns/ini_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
+
 package dns
 
 import (