virtio_input: Constify id_table

id_table is not modified, so make it const to allow the compiler to put
it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200911203509.26505-3-rikard.falkeborn@gmail.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>