virtio_9p.h: include linux/types.h

Add <linux/types.h> to <linux/virtio_9p.h> so that types are explicitly
defined:
linux/virtio_9p.h:15: found __[us]{8,16,32,64} type without #include <linux/types.h>

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>