Add require for ClassAttribute in class/attribute core extension

rails/rails@1962e8d introduced the ClassAttribute class. We need to require it in the core_ext/class/attribute or else we will get a const missing error in gems or apps that are only requiring that core extension and not all of active support.

Author: andrewn617

activesupport/lib/active_support/core_ext/class/attribute.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/module/redefine_method"
+require "active_support/class_attribute"
 
 class Class
   # Declare a class-level attribute whose value is inheritable by subclasses.