x86 mpparse: build fix

fix this build bug:

 drivers/acpi/pci_irq.c: In function 'acpi_pci_irq_enable':
 drivers/acpi/pci_irq.c:574: error: implicit declaration of function 'mp_config_acpi_gsi'

Signed-off-by: Ingo Molnar <mingo@elte.hu>