iommufd/selftest: Make dirty_ops static

iommufd/selftest: Make dirty_ops static

The sparse tool complains as follows:

drivers/iommu/iommufd/selftest.c:277:30: warning:
	symbol 'dirty_ops' was not declared. Should it be static?

This symbol is not used outside of selftest.c, so marks it static.

Fixes: 266ce58989ba ("iommufd/selftest: Test IOMMU_HWPT_ALLOC_DIRTY_TRACKING")
Link: https://patch.msgid.link/r/20240819120007.3884868-1-ruanjinjie@huawei.com
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>