net: Provide stub for __netif_set_xps_queue if there is no CONFIG_XPS

Building virtio_net driver without CONFIG_XPS fails with:

    drivers/net/virtio_net.c: In function ‘virtnet_set_affinity’:
    drivers/net/virtio_net.c:1910:3: error: implicit declaration of function ‘__netif_set_xps_queue’ [-Werror=implicit-function-declaration]
       __netif_set_xps_queue(vi->dev, mask, i, false);
       ^
Fixes: 4d99f660 ("net: allow to call netif_reset_xps_queues() under cpus_read_lock")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>