xen/spinlock: Don't setup xen spinlock IPI kicker if disabled.

There is no need to setup this kicker IPI if we are never going
to use the paravirtualized ticketlock mechanism.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>