x86/xen/apic: Add missing #include <xen/xen.h>

This file depends on <xen/xen.h>, but the dependency was hidden due
to: <asm/acpi.h> -> <asm/trampoline.h> -> <asm/io.h> -> <xen/xen.h>

With the removal of <asm/trampoline.h>, this exposed the missing

Cc: Len Brown <lenb@kernel.org>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Link: http://lkml.kernel.org/n/tip-7ccybvue6mw6wje3uxzzcglj@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/xen/apic.c

 #include <linux/init.h>
+
 #include <asm/x86_init.h>
 #include <asm/apic.h>
-#include <xen/interface/physdev.h>
 #include <asm/xen/hypercall.h>
 
+#include <xen/xen.h>
+#include <xen/interface/physdev.h>
+
 unsigned int xen_io_apic_read(unsigned apic, unsigned reg)
 {
 	struct physdev_apic apic_op;