virtio-net: fix build error when CONFIG_AVERAGE is not enabled

Commit ab7db917 ("virtio-net: auto-tune mergeable rx buffer size for
improved performance") introduced a virtio-net dependency on EWMA.
The inclusion of EWMA is controlled by CONFIG_AVERAGE. Fix build error
when CONFIG_AVERAGE is not enabled by adding select AVERAGE to
virtio-net's Kconfig entry.

Build failure reported using config make ARCH=s390 defconfig.
Signed-off-by: Michael Dalton <mwdalton@google.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/Kconfig

@@ -236,6 +236,7 @@ config VETH
 config VIRTIO_NET
 	tristate "Virtio network driver"
 	depends on VIRTIO
+	select AVERAGE
 	---help---
 	  This is the virtual network driver for virtio.  It can be used with
 	  lguest or QEMU based VMMs (like KVM or Xen).  Say Y or M.