x86, AMD IOMMU: build fix

fix:

 arch/x86/kernel/amd_iommu_init.c:247: warning: 'struct acpi_table_header' declared inside parameter list
 arch/x86/kernel/amd_iommu_init.c:247: warning: its scope is only this definition or declaration, which is probably not what you want
 arch/x86/kernel/amd_iommu_init.c: In function 'find_last_devid_acpi':
 arch/x86/kernel/amd_iommu_init.c:257: error: dereferencing pointer to incomplete type
 arch/x86/kernel/amd_iommu_init.c:265: error: dereferencing pointer to incomplete type

the AMD IOMMU code depends on ACPI facilities.

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