Fix ForkTracker on ruby <= 2.5.3

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.

The possible fix on 2.5.4 is 75aba10d7a

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

activesupport/lib/active_support/fork_tracker.rb

@@ -20,7 +20,11 @@ def fork(*)
 
     module CoreExtPrivate
       include CoreExt
-      private :fork
+
+      private
+        def fork(*)
+          super
+        end
     end
 
     @pid = Process.pid