Fix ForkTracker on ruby <= 2.5.3

[p8]

Summary

Making the fork method private by calling private :fork raises a
"no superclass method `fork'" error when calling super in a subclass on
ruby <= 2.5.3. The error doesn't occur on ruby 2.5.4 and higher.
Making the method private by redefining doesn't raise the error.

Other Information

The possible fix on 2.5.4 is ruby/ruby@75aba10

The error can be reproduced with the following script on ruby 2.5.3:

class Cluster
  def start
    fork { puts "forked!" }
  end
end

module CoreExt
  def fork(*)
    super
  end
end

module CoreExtPrivate
  include CoreExt
  private :fork
end

::Object.prepend(CoreExtPrivate)
Cluster.new.start

Fixes #40603