[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-markupSigned-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/rtnetlink.h

@@ -199,6 +199,7 @@ enum
 #define RTPROT_BIRD	12	/* BIRD */
 #define RTPROT_DNROUTED	13	/* DECnet routing daemon */
 #define RTPROT_XORP	14	/* XORP */
+#define RTPROT_NTK	15	/* Netsukuku */
 
 /* rtm_scope