fix Role#find_or_create_system_role is not private

git-svn-id: http://svn.redmine.org/redmine/trunk@18693 e93f8b46-1217-0410-a6f0-8f06a7374b81

app/models/role.rb

@@ -304,4 +304,5 @@ private
     end
     role
   end
+  private_class_method :find_or_create_system_role
 end