iommu: Add missing forward declaration in include file

The 'struct notifier_block' is not used in linux/iommu.h but
not declared anywhere. Add a forward declaration for it.
Reported-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>

include/linux/iommu.h

@@ -31,6 +31,7 @@ struct iommu_group;
 struct bus_type;
 struct device;
 struct iommu_domain;
+struct notifier_block;
 
 /* iommu fault flags */
 #define IOMMU_FAULT_READ	0x0