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>