MDL-77350 cache: 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.

cache/classes/dummystore.php

@@ -59,6 +59,12 @@ class cachestore_dummy extends cache_store {
      */
     protected $store = array();
 
+    /**
+     * Cache definition
+     * @var cache_definition
+     */
+    protected $definition;
+
     /**
      * Constructs a dummy store instance.
      * @param string $name
@@ -119,6 +125,8 @@ class cachestore_dummy extends cache_store {
         } else {
             $this->persist = !$definition->use_static_acceleration();
         }
+
+        $this->definition = $definition;
     }
 
     /**