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: b398f91 ("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>

Author: arndb

include/linux/vfio.h

@@ -74,13 +74,11 @@ struct vfio_device {
 	u8 iommufd_attached:1;
 #endif
 	u8 cdev_opened:1;
-#ifdef CONFIG_DEBUG_FS
 	/*
 	 * debug_root is a static property of the vfio_device
 	 * which must be set prior to registering the vfio_device.
 	 */
 	struct dentry *debug_root;
-#endif
 };
 
 /**