[NET] rtnetlink: Add RTPROT entry for Netsukuku.

The Netsukuku daemon is using the same number to mark its routes, you
can see it here:
http://hinezumilabs.org/cgi-bin/viewcvs.cgi/netsukuku/src/krnl_route.h?rev=HEAD&content-type=text/vnd.viewcvs-markup



Signed-off-by: David S. Miller <davem@davemloft.net>