LdapUserGroupBackend: Do not consider every "member" as a "user"

Not all members of a group are actual user objects. I would have liked to
actually only show real users, but this is currently not possible.

refs #9772

library/Icinga/Authentication/UserGroup/LdapUserGroupBackend.php

@@ -467,7 +467,7 @@ class LdapUserGroupBackend extends LdapRepository implements UserGroupBackendInt
     {
         return $this->ds
             ->select()
-            ->from($this->userClass, array($this->userNameAttribute))
+            ->from('*', array($this->userNameAttribute))
             ->setBase($dn)
             ->fetchOne();
     }