[ISDN]: Fix compile with CONFIG_ISDN_X25 disabled.

On Mon, Oct 15, 2007 at 06:44:56PM +0400, Denis V. Lunev wrote:
Compilation fix. The problem appears after
7c076d1de869256848dacb8de0050a3a390f95df by Karsten Keil <kkeil@suse.de>

Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>