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: NIngo Molnar <mingo@elte.hu>

arch/x86/Kconfig

@@ -552,7 +552,7 @@ config CALGARY_IOMMU_ENABLED_BY_DEFAULT
 config AMD_IOMMU
 	bool "AMD IOMMU support"
 	select SWIOTL
-	depends on X86_64 && PCI
+	depends on X86_64 && PCI && ACPI
 	help
 	  Select this to get support for AMD IOMMU hardware in your system.