iommu/hyperv: Make hv_iommu_device static

Make hv_iommu_device static to give it internal linkage. This
pointer is only used within hv-iommu-guest.c, but previously
has external linkage without a prior declaration. This causes
sparse to warn that the symbol was not declared.

Fixes: 2690d4156b51 ("iommu/hyperv: Add para-virtualized IOMMU support for Hyper-V guest")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202610092126.TuSXnTYR-lkp@intel.com/
Signed-off-by: Yu Zhang <zhangyu1@linux.microsoft.com>
Reviewed-by: Jacob Pan <jacob.pan@linux.microsoft.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>