iommufd: Move struct iommufd_object to public iommufd header

iommufd: Move struct iommufd_object to public iommufd header

Prepare for an embedded structure design for driver-level iommufd_viommu
objects:
    // include/linux/iommufd.h
    struct iommufd_viommu {
        struct iommufd_object obj;
        ....
    };

    // Some IOMMU driver
    struct iommu_driver_viommu {
        struct iommufd_viommu core;
        ....
    };

It has to expose struct iommufd_object and enum iommufd_object_type from
the core-level private header to the public iommufd header.

Link: https://patch.msgid.link/r/54a43b0768089d690104530754f499ca05ce0074.1730836219.git.nicolinc@nvidia.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>