io/console: vk_header only on Windows
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50428 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io/console: vk_header only on Windows
nonsense to update on other platforms.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@50428 b2dd03c8-39d4-4d8f-98ff-823fe69b080e