Add require_once to update.php due to routing

Fixes https://github.com/owncloud/core/issues/10585
Partially reverts 52d5429768acdb87b2dc2efedc89eb4ad0d29139

Testplan
-----------

1. Setup OC on stable7
2. Switch to the "fix-10585" branch
3. Run the upgrade

@PVince81