fix: restore ObjectStructureElement import in Category

Author: SMillerDev

src/PHPDraft/Model/Category.php

@@ -13,6 +13,7 @@
 namespace PHPDraft\Model;
 
 use PHPDraft\Model\Elements\BasicStructureElement;
+use PHPDraft\Model\Elements\ObjectStructureElement;
 
 /**
  * Class Category.