Rebuild permissions on cache clear

After installing a new Drupal site with a custom profile enabling many contrib modules, I found an administrator account was lacking many necessary privileges. I looked at adminrole.module and found it only rebuilds permissions when submitting the modules page (which suggests it will also not rebuild when modules are enabled/disabled via drush, which is our primary management tool).

See also #785246: CCK content type import and #753606: Support "Field Permissions" module... I can't say for sure, but I would imagine any scenario that would require rebuilding permissions would flush the caches, so I think it would be best for adminrole to do the rebuild on a cache clear. Patch submitted, tested with drush cc all.