MDL-77338 backup: 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.

backup/controller/base_controller.class.php

@@ -43,6 +43,9 @@ abstract class base_controller extends backup implements loggable {
      */
     protected $copy;
 
+    /** @var int Backup mode. */
+    protected $mode;
+
     /**
      * Gets the progress reporter, which can be used to report progress within
      * the backup or restore process.