iommu: Add empty stub for iommu_group_get_by_id()

Almost every function in include/linux/iommu.h has an empty stub
but the iommu_group_get_by_id() did not get one by mistake.

This adds an empty stub for iommu_group_get_by_id() for IOMMU_API
disabled config.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Joerg Roedel <joro@8bytes.org>