add a missing include for the WP_XML client

includes/class-syndication-client-factory.php

 <?php
 
 include_once( dirname( __FILE__ ) . '/class-syndication-wp-xmlrpc-client.php' );
+include_once( dirname( __FILE__ ) . '/class-syndication-wp-xml-client.php' );
 include_once( dirname( __FILE__ ) . '/class-syndication-wp-rest-client.php' );
 include_once( dirname( __FILE__ ) . '/class-syndication-wp-rss-client.php' );
 
@@ -43,4 +44,4 @@ class Syndication_Client_Factory {
 		return 'Syndication_' . $transport_type . '_Client';
 	}
 
-}
\ No newline at end of file
+}