iommu/dma: Fix function declaration

Newly added code in the ipmmu-vmsa driver showed a small mistake
in a header file that can't be included by itself without CONFIG_IOMMU_DMA
enabled:

In file included from drivers/iommu/ipmmu-vmsa.c:13:0:
include/linux/dma-iommu.h:105:94: error: 'struct device' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]

This adds a forward declaration for 'struct device', similar to how
we treat the other struct types in this case.

Fixes: 3ae47292 ("iommu/ipmmu-vmsa: Add new IOMMU_DOMAIN_DMA ops")
Fixes: 273df963 ("iommu/dma: Make PCI window reservation generic")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>