Fix a warning in of_iommu.c:

drivers/iommu/of_iommu.c:38:5: warning: no previous prototype for 'of_get_dma_window' [-Wmissing-prototypes]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Hiroshi DOYU <hdoyu@nvidia.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>