LDAP: fix background job, fixes #8995

A parameter was missing to instantiate Access in the background job. 

To test:
- have LDAP configured, but with only one active server configuration!
- run cron.php (mind interval  check, temporarily set it to  0 in https://github.com/owncloud/core/blob/61f6ca25a8f1430a4bc881388caa091a4de040c0/apps/user_ldap/lib/jobs.php#L68)
 - before: error as described in https://github.com/owncloud/core/issues/8995
 - after: works 

Please test/review @paszczus @PVince81 @Xenopathic or others, ty