fix visibility of SecureRandom.gen_random

	Aliasing a method preserves its visibility.  These aliases turn
	formerly-public methods into private.  Should make them public
	again.  [Bug #15847]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67723 b2dd03c8-39d4-4d8f-98ff-823fe69b080e