MAINTAINERS: add vringh.h to Virtio Core and Net Drivers

vringh.h doesn't seem to belong to any section in MAINTAINERS.
Add it to Virtio Core and Net Drivers, which seems to be the most
appropriate section to me.

Signed-off-by: Simon Horman <horms@kernel.org>
Message-Id: <20230331-vhost-fixes-v1-3-1f046e735b9e@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>