spawn_spec.rb: add missing platform guard
spawn_spec.rb: add missing platform guard

this exists for `context "when passed close_others: false" do`, but this
seems to be missing for `context "when passed close_others: true"`.
And this seems to hang forever on Windows since r66622.
https://ci.appveyor.com/project/ruby/ruby/builds/21277729

git-svn-id: ab86ecd26fe50a6a239cacb71380e346f71cee7d@66627 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

nobu/rubyspec,ruby/spec,eregon/rubyspec,eregon/rubyspec,ruby/rubyspec,ruby/spec,ruby/rubyspec,nobu/rubyspec,eregon/rubyspec,nobu/rubyspec,ruby/spec