iommufd/iommufd_private.h: Avoid -Wflex-array-member-not-at-end warning

iommufd/iommufd_private.h: Avoid -Wflex-array-member-not-at-end warning

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Move the conflicting declaration to the end of the corresponding
structure. Notice that struct iommufd_vevent is a flexible
structure, this is a structure that contains a flexible-array
member.

Fix the following warning:

drivers/iommu/iommufd/iommufd_private.h:621:31: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Link: https://patch.msgid.link/r/aRHOAwpATIE0oajj@kspp
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Fixes: e36ba5ab808e ("iommufd: Add IOMMUFD_OBJ_VEVENTQ and IOMMUFD_CMD_VEVENTQ_ALLOC")
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>