x86, AMD IOMMU: add amd_iommu_init.c to Makefile

This patch adds the source file amd_iommu_init.c to the kernel Makefile for the
x86 architecture.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: iommu@lists.linux-foundation.org
Cc: bhavna.sarathy@amd.com
Cc: Sebastian.Biemueller@amd.com
Cc: robert.richter@amd.com
Cc: joro@8bytes.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>