Fix undefined constant error for STATUS_SERVICE_UNAVAILABLE

Signed-off-by: Julius Härtl <jus@bitgrid.net>

ocs/v1.php

 	// since the behavior of apps or remotes are unpredictable during
 	// an upgrade, return a 503 directly
 	http_response_code(503);
-	$response = new \OC\OCS\Result(null, OC_Response::STATUS_SERVICE_UNAVAILABLE, 'Service unavailable');
+	$response = new \OC\OCS\Result(null, 503, 'Service unavailable');
 	OC_API::respond($response, OC_API::requestedFormat());
 	exit;
 }