virtio_balloon: not EXPERIMENTAL any more.

It is not experimental in any vaguely-sane sense.
Reported-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/virtio/Kconfig

@@ -26,8 +26,8 @@ config VIRTIO_PCI
 	  If unsure, say M.
 
 config VIRTIO_BALLOON
-	tristate "Virtio balloon driver (EXPERIMENTAL)"
-	depends on EXPERIMENTAL && VIRTIO
+	tristate "Virtio balloon driver"
+	depends on VIRTIO
 	---help---
 	 This driver supports increasing and decreasing the amount
 	 of memory within a KVM guest.