Add CustomerGroupObject instead of GroupObject

I found when I logged into the customer interface these error messages:

  Message: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 6, SQL: 'SELECT ru.role_id, ru.user_id, r.name, u.login FROM role_user ru, roles r, users u, referer: http://localhost/otrs/customer.pl?Action=Logout
         WHERE r.valid_id IN (1), referer: http://localhost/otrs/customer.pl?Action=Logout
             AND r.id = ru.role_id, referer: http://localhost/otrs/customer.pl?Action=Logout
             AND ru.user_id = u.id, referer: http://localhost/otrs/customer.pl?Action=Logout
             AND u.valid_id IN (1), referer: http://localhost/otrs/customer.pl?Action=Logout
             AND ', referer: http://localhost/otrs/customer.pl?Action=Logout
 , referer: http://localhost/otrs/customer.pl?Action=Logout

This was due to the fact that the GroupObject was used instead of the
CustomerGroupObject! This commit fixes it.