x86, intel-iommu: fix X2APIC && !ACPI build failure

This build failure:

| drivers/pci/dmar.c:47: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘dmar_tbl_size’
| drivers/pci/dmar.c:62: warning: ‘struct acpi_dmar_device_scope’ declared inside parameter list
| drivers/pci/dmar.c:62: warning: its scope is only this definition or declaration, which is probably not what you want

Triggers due to this commit:

  d0b03bd1

: x2apic/intr-remap: decouple interrupt remapping from x2apic

Which exposed a pre-existing but dormant fragility of the 'select X86_X2APIC'
it moved around and turned that fragility into a build failure.

Replace it with a proper 'depends on' construct.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
LKML-Reference: <1239084280.22733.404.camel@macbook.infradead.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>