Correctly redirect to settings page when not selecting cert file for upload

Fix #5463

apps/files_external/ajax/addRootCertificate.php

@@ -4,7 +4,7 @@ OCP\JSON::checkAppEnabled('files_external');
 OCP\JSON::callCheck();
 
 if ( ! ($filename = $_FILES['rootcert_import']['name']) ) {
-	header("Location: settings/personal.php");
+	header('Location:' . OCP\Util::linkToRoute( "settings_personal" ));
 	exit;
 }