wgengine/winnet: don't build on non-windows

It only affects 'go install ./...', etc, and only on darwin/arm64 (M1 Macs) where
the go-ole package doesn't compile.

No need to build it.

Updates #943

wgengine/winnet/winnet.go

@@ -2,13 +2,16 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build windows
+
 package winnet
 
 import (
 	"fmt"
+	"unsafe"
+
 	"github.com/go-ole/go-ole"
 	"github.com/go-ole/go-ole/oleutil"
-	"unsafe"
 )
 
 const CLSID_NetworkListManager = "{DCB00C01-570F-4A9B-8D69-199FDBA5723B}"