intel-iommu: Fix one last ia64 build problem in Pass Through Support

On ia64 with CONFIG_DMAR=n and CONFIG_SWIOTLB=y (as used in
arch/ia64/configs/tiger_defconfig) there is still a link
error with iommu_pass_through listed as an undefined symbol:

arch/ia64/kernel/built-in.o: In function `pci_swiotlb_init':
(.init.text+0x7f70): undefined reference to `iommu_pass_through'

Fix it by #defining iommu_pass_through away in asm/iommu.h

Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>