vfio: unhide vdev->debug_root

When debugfs is disabled, the hisilicon driver now fails to build:

drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c: In function 'hisi_acc_vfio_debug_init':
drivers/vfio/pci/hisilicon/hisi_acc_vfio_pci.c:1671:62: error: 'struct vfio_device' has no member named 'debug_root'
1671 | vfio_dev_migration = debugfs_lookup("migration", vdev->debug_root);
| ^~

The driver otherwise relies on dead-code elimination, but this reference
fails. The single struct member is not going to make much of a difference
for memory consumption, so just keep this visible unconditionally.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b398f91779b8 ("hisi_acc_vfio_pci: register debugfs for hisilicon migration driver")
Link: https://lore.kernel.org/r/20260327165521.3779707-1-arnd@kernel.org
Signed-off-by: Alex Williamson <alex@shazbot.org>