iommu/vt-d: Make domain attach helpers to be extern

This makes the helpers visible to nested.c.

Link: https://lore.kernel.org/r/20231026044216.64964-6-yi.l.liu@intel.com
Suggested-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>