util: include virtio UAPI headers in sync

We already have an update_headers.sh sync script, where we occasionally
update the KVM interface UAPI kernel headers into our tree.
So far this covered only the generic kvm.h, plus each architecture's
version of that file.
Commit  bc77bf49 ("stat: Add descriptions for new virtio_balloon
stat types") used newer virtio symbols, which some older distros do not
include in their kernel headers package. To help fixing this and to
avoid similar problems in the future, add the virtio headers to our sync
script, so that we can get the same, up-to-date versions of the headers
easily.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/20220524150611.523910-3-andre.przywara@arm.com


Signed-off-by: Will Deacon <will@kernel.org>