fix `wintun` package to have // +build comments (#598)

Without these comments, gofmt 1.16.9 will complain. Since we otherwise
still support building with go1.16, lets add the comments to make it
easier to compile and gofmt.

Related: #588

wintun/device.go

@@ -1,4 +1,5 @@
 //go:build windows
+// +build windows
 
 /* SPDX-License-Identifier: MIT
  *

wintun/tun.go

@@ -1,4 +1,5 @@
 //go:build windows
+// +build windows
 
 /* SPDX-License-Identifier: MIT
  *