vfio/iommu_type1: Fix duplicate included kthread.h

linux/kthread.h is included more than once, remove the one that isn't
necessary.

Fixes: 898b9eae ("vfio/type1: block on invalid vaddr")
Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>