MDL-77350 completion: Added class properties that are not declared

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

completion/data_object.php

@@ -64,6 +64,9 @@ abstract class data_object {
     /* @var int The primary key */
     public $id;
 
+    /** @var int completed status. */
+    public $completedself;
+
 
     /**
      * Constructor. Optionally (and by default) attempts to fetch corresponding row from DB.