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>
diff --git a/include/linux/iommu.h b/include/linux/iommu.h
index 0a85199..7e83370 100644
--- a/include/linux/iommu.h
+++ b/include/linux/iommu.h
@@ -31,6 +31,7 @@
 struct bus_type;
 struct device;
 struct iommu_domain;
+struct notifier_block;
 
 /* iommu fault flags */
 #define IOMMU_FAULT_READ	0x0