vhost: fix TUN=m VHOST_NET=y

drivers/built-in.o: In function `get_tun_socket':
    net.c:(.text+0x15436e): undefined reference to `tun_get_socket'

If tun is a module, vhost must be a module, too.
If tun is built-in or disabled, vhost can be built-in.

Note: TUN || !TUN might look a bit strange until you realize
that boolean logic rules do not apply for tristate variables.
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/vhost/Kconfig

 config VHOST_NET
 	tristate "Host kernel accelerator for virtio net (EXPERIMENTAL)"
-	depends on NET && EVENTFD && EXPERIMENTAL
+	depends on NET && EVENTFD && (TUN || !TUN) && EXPERIMENTAL
 	---help---
 	  This kernel module can be loaded in host kernel to accelerate
 	  guest networking with virtio_net. Not to be confused with virtio_net