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>

include/linux/iommu.h

@@ -247,6 +247,11 @@ static inline struct iommu_domain *iommu_domain_alloc(struct bus_type *bus)
 	return NULL;
 }
 
+static inline struct iommu_group *iommu_group_get_by_id(int id)
+{
+	return NULL;
+}
+
 static inline void iommu_domain_free(struct iommu_domain *domain)
 {
 }