iommu/core: Fix build with INTR_REMAP=y && CONFIG_DMAR=n

IOMMU_API is not selected when no DMA remapping driver is
selected, but the whole drivers/iommu/ directory is only
built with IOMMU_API=y. Fixed with this patch by including
the directory with IOMMU_SUPPORT instead.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>