Add missing schema

Author: swissspidy

phpunit.xml.dist

@@ -1,10 +1,13 @@
+<?xml version="1.0" ?>
 <phpunit
 	bootstrap="tests/bootstrap.php"
 	colors="true"
 	convertErrorsToExceptions="true"
 	convertWarningsToExceptions="true"
 	convertNoticesToExceptions="true"
 	convertDeprecationsToExceptions="true"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/8.5/phpunit.xsd"
 	>
 	<testsuites>
 		<testsuite name="default">