Add Open Routing Protocol ID to `rtnetlink.h`

- The Open Routing (Open/R) network protocol netlink handler uses ID 99
- Will also add to `/etc/iproute2/rt_protos` once this is accepted
- For more information: https://github.com/facebook/openr
Signed-off-by: From: Cooper Lees <me@cooperlees.com>
Signed-off-by: David S. Miller <davem@davemloft.net>