iommufd/selftest: Set varaiable mock_iommu_device storage-class-specifier to static

iommufd/selftest: Set varaiable mock_iommu_device storage-class-specifier to static

smatch reports:

drivers/iommu/iommufd/selftest.c:295:21: warning: symbol
  'mock_iommu_device' was not declared. Should it be static?

This variable is only used in one file so it should be static.

Fixes: 65c619ae0680 ("iommufd/selftest: Make selftest create a more complete mock device")
Link: https://lore.kernel.org/r/20230404002317.1912530-1-trix@redhat.com
Signed-off-by: Tom Rix <trix@redhat.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>