OG's default permissions not set when using Drush

When installing a site with Drush, hook_modules_enabled can be fired multiple times, with different modules available each time. This means different permissions are defined each time, but the use of a static variable in og_get_permissions() means permissions are not reloaded when a new module is enabled. This means that default permissions are not correctly set up on install.

In other words, that static variable needs resetting. The attached patch does this :-)