vfio/iova_bitmap: Explicitly include linux/slab.h

kzalloc/kzfree are used so include `slab.h`. While it happens to work
without it, due to commit 8b9f3ac5

 ("fs: introduce alloc_inode_sb() to
allocate filesystems specific inode") which indirectly includes via:

. ./include/linux/mm.h
.. ./include/linux/huge_mm.h
... ./include/linux/fs.h
.... ./include/linux/slab.h

Make it explicit should any of its indirect dependencies be dropped/changed
for entirely different reasons as it was the cause prior to commit above
recently (i.e. <= v5.18).

Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Link: https://lore.kernel.org/r/20221025193114.58695-2-joao.m.martins@oracle.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>