🐛 Fix undefined local variable iiif_print

There was a change in this version of IIIF Print that needed an update
to routes.rb.

Ref:
  - notch8/iiif_print#302

Author: Kirk Wang

config/routes.rb

@@ -11,6 +11,7 @@
   concern :oai_provider, BlacklightOaiProvider::Routes.new
 
   mount Hyrax::IiifAv::Engine, at: '/'
+  mount IiifPrint::Engine, at: '/'
   mount Riiif::Engine => 'images', as: :riiif if Hyrax.config.iiif_image_server?
 
   authenticate :user, ->(u) { u.superadmin? || u.admin? } do