prelude.rb (Kernel#pp): make it private

It was a bug.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64994 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

prelude.rb

@@ -151,4 +151,6 @@ module Kernel
 
   # suppress redefinition warning
   alias pp pp # :nodoc:
+
+  private :pp
 end