missing dependency for virtio_mmio

Driver really depends on having readb/writeb/etc. - won't even build on things like s390 or uml and on tile without PCI it'll build but panic if you manage to run it...

---

index 816ed08..4ae52fc 100644
@@ -37,7 +37,7 @@ config VIRTIO_BALLOON
 
  config VIRTIO_MMIO
  	tristate "Platform bus driver for memory mapped virtio devices (EXPERIMENTAL)"
- 	depends on EXPERIMENTAL
+ 	depends on EXPERIMENTAL && HAS_IOMEM
  	select VIRTIO
  	select VIRTIO_RING
  	---help---

Thanks, already queued. I was about to send it when YA virtio fix turned up, and I like them to sit in linux-next for a cycle first.

That said, happy for Linus to take this now if it's breaking people.

Cheers,
Rusty.