xen-acpi-processor: Add missing #include <xen/xen.h>

xen-acpi-processor: 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

Reported-by: Ingo Molnar <[email protected]>
Cc: Len Brown <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Jeremy Fitzhardinge <[email protected]>
Cc: Jarkko Sakkinen <[email protected]>
Signed-off-by: H. Peter Anvin <[email protected]>