sync IO when required, instead of only when using extend

When using nrpe_check include (not extend, see below), IO is not sync'd properly. 

This causes NRPE to not read the output generated in the check block. This PR enables fsync for both usage styles (include/extend).

    require 'nrpe_check'
    class CheckStuff
      include NRPE::Check
      def initialize
      end
      def run
        check do
          status :unknown, "unknown"
          exit_with_status :ok, "groovy"
        end
      end
    end
    CheckStuff.new.run