Revert const visibility modifier

Author: sidolov

app/code/Magento/Downloadable/Observer/IsAllowedGuestCheckoutObserver.php

@@ -16,7 +16,7 @@ class IsAllowedGuestCheckoutObserver implements ObserverInterface
     /**
      *  Xml path to disable checkout
      */
-    private const XML_PATH_DISABLE_GUEST_CHECKOUT = 'catalog/downloadable/disable_guest_checkout';
+    const XML_PATH_DISABLE_GUEST_CHECKOUT = 'catalog/downloadable/disable_guest_checkout';
 
     /**
      *  Xml path to get downloadable Shareable setting