x86/xen: cleanup includes in arch/x86/xen/spinlock.c

arch/x86/xen/spinlock.c includes several headers which are not needed.
Remove the #includes.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>