Require sprockets deprecator in Railtie

With the release of sprockets-rails 3.5.0, our local test suite breaks
with

```
NoMethodError:
  undefined method `deprecator' for Sprockets::Rails:Module
 /home/circleci/solidus/vendor/bundle/ruby/3.2.0/gems/sprockets-rails-3.5.0/lib/sprockets/railtie.rb:129:in `block in <class:Railtie>'
```

This should fix that by explicitly requiring the
`Sprockets::Rails.deprecator` in both `lib/sprockets/rails.rb` and
`lib/sprockets/railtie.rb`.

Should fix #524

lib/sprockets/rails.rb

@@ -3,11 +3,9 @@
 if defined? Rails::Railtie
   require 'sprockets/railtie'
 end
+require 'sprockets/rails/deprecator'
 
 module Sprockets
   module Rails
-    def self.deprecator
-      @deprecator ||= ActiveSupport::Deprecation.new("4.0", "Sprockets::Rails")
-    end
   end
 end

lib/sprockets/rails/deprecator.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module Sprockets
+  module Rails
+    def self.deprecator
+      @deprecator ||= ActiveSupport::Deprecation.new("4.0", "Sprockets::Rails")
+    end
+  end
+end

lib/sprockets/railtie.rb

@@ -6,6 +6,7 @@
 require 'sprockets'
 
 require 'sprockets/rails/asset_url_processor'
+require 'sprockets/rails/deprecator'
 require 'sprockets/rails/sourcemapping_url_processor'
 require 'sprockets/rails/context'
 require 'sprockets/rails/helper'