xen: make CPU hotplug functions static

There's no need for these functions to be accessed from outside of xen/smp.c

Signed-off-by: Alex Nixon <alex.nixon@citrix.com>
Acked-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/xen/xen-ops.h

@@ -51,10 +51,6 @@ void xen_mark_init_mm_pinned(void);
 
 void __init xen_setup_vcpu_info_placement(void);
 
-void xen_play_dead(void);
-void xen_cpu_die(unsigned int cpu);
-int xen_cpu_disable(void);
-
 #ifdef CONFIG_SMP
 void xen_smp_init(void);