iommu: Fix prototype of iommu_paging_domain_alloc_flags()

The iommu_paging_domain_alloc_flags() prototype for
non-iommu kernel configurations lacks the 'static inline'
prefixes.

Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Vasant Hegde <vasant.hegde@amd.com>
Fixes: 20858d4e ("iommu: Introduce iommu_paging_domain_alloc_flags()")
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>