MDL-77343 mod_assign: Added missing class properties.

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.

mod/assign/gradingtable.php

@@ -64,6 +64,8 @@ class assign_grading_table extends table_sql implements renderable {
     private $plugincache = array();
     /** @var array $scale - A list of the keys and descriptions for the custom scale */
     private $scale = null;
+    /** @var bool true if the user has this capability. Otherwise false. */
+    private $hasviewblind;
 
     /**
      * overridden constructor keeps a reference to the assignment class that is displaying this table