Xen: do hv callback accounting only on x86

Patch 99c8b79d3c1 "xen: Add proper irq accounting for HYPERCALL vector"
added a call to inc_irq_stat(irq_hv_callback_count) in common Xen code,
however both the inc_irq_stat function and the irq_hv_callback_count
counter are architecture specific.

This makes the code build again on ARM by moving the call into the
existing #ifdef CONFIG_X86. We may want to later do the same implementation
on ARM that x86 has though.

Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: David Vrabel <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Konrad Rzeszutek Wilk <[e-mail]>
Cc: Xen <[e-mail]>

drivers/xen/events/events_base.c

···
 	irq_enter();
 #ifdef CONFIG_X86
 	exit_idle();
-#endif
 	inc_irq_stat(irq_hv_callback_count);
+#endif
 
 	__xen_evtchn_do_upcall();