This PR fixes the following error that showed up after upgrading to 1.31.
```
Array ( [0] => 1 [1] => Uncaught Error: Call to undefined function init_auth() in /srv/deployment/librenms/librenms-cache/revs/8c9da11850ecbf2a949b9317d1fce75301bfd13b/includes/init.php:152 Stack trace: #0 /srv/deployment/librenms/librenms-cache/revs/8c9da11850ecbf2a949b9317d1fce75301bfd13b/html/index.php(59): require() #1 {main} thrown [2] => /srv/deployment/librenms/librenms-cache/revs/8c9da11850ecbf2a949b9317d1fce75301bfd13b/includes/init.php [3] => 152 )
```
it's most likeley related to the fact that we use a symlink, and $config['install_dir'] is the symlink, and $install_dir is the real folder.