xen: add a missing #include to arch/x86/pci/xen.c

Add missing #include <asm/io_apic.h> to arch/x86/pci/xen.c.
Signed-off-by: NStefano Stabellini <stefano.stabellini@eu.citrix.com>

arch/x86/pci/xen.c

@@ -10,6 +10,7 @@
 #include <linux/acpi.h>
 
 #include <linux/io.h>
+#include <asm/io_apic.h>
 #include <asm/pci_x86.h>
 
 #include <asm/xen/hypervisor.h>