iommu/vt-d: Set variable intel_dirty_ops to static

Fix the following warning:
drivers/iommu/intel/iommu.c:302:30: warning: symbol
 'intel_dirty_ops' was not declared. Should it be static?

This variable is only used in its defining file, so it should be static.

Fixes: f35f22cc

 ("iommu/vt-d: Access/Dirty bit support for SS domains")
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20231120101025.1103404-1-chentao@kylinos.cn
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>