Allow XML entity loader for MDB2 schema loader

Forward port of 762b0d9 from stable6 to master

lib/private/db/mdb2schemareader.php

@@ -41,7 +41,9 @@ class MDB2SchemaReader {
 	 */
 	public function loadSchemaFromFile($file) {
 		$schema = new \Doctrine\DBAL\Schema\Schema();
+		$loadEntities = libxml_disable_entity_loader(false);
 		$xml = simplexml_load_file($file);
+		libxml_disable_entity_loader($loadEntities);
 		foreach ($xml->children() as $child) {
 			/**
 			 * @var \SimpleXMLElement $child