x86, AMD IOMMU: build fix #3

fix typo causing:

arch/x86/kernel/built-in.o: In function `__unmap_single':
amd_iommu.c:(.text+0x17771): undefined reference to `iommu_area_free'
arch/x86/kernel/built-in.o: In function `__map_single':
amd_iommu.c:(.text+0x1797a): undefined reference to `iommu_area_alloc'
amd_iommu.c:(.text+0x179a2): undefined reference to `iommu_area_alloc'
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/Kconfig

@@ -551,7 +551,7 @@ config CALGARY_IOMMU_ENABLED_BY_DEFAULT
 
 config AMD_IOMMU
 	bool "AMD IOMMU support"
-	select SWIOTL
+	select SWIOTLB
 	depends on X86_64 && PCI && ACPI
 	help
 	  Select this to get support for AMD IOMMU hardware in your system.