drivers/pci/intr_remapping.c: include acpi.h

Fix this build error:
drivers/pci/intr_remapping.c: In function 'ir_parse_ioapic_scope':
drivers/pci/intr_remapping.c:617: error: invalid use of undefined type
				         'struct acpi_dmar_hardware_unit'

Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>