virtio_net: add _UAPI prefix to virtio_net header guards

This gives better namespacing and prevents conflicts with no-uapi version
of virtio_net header that will be introduced in the following patch.

Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>