sockopt: add support for querying tcp round-trip-time

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/sockopt.h b/lib/sockopt.h
index aced6d4..cb14efc 100644
--- a/lib/sockopt.h
+++ b/lib/sockopt.h
@@ -100,6 +100,7 @@
 extern void sockopt_iphdrincl_swab_htosys (struct ip *iph);
 extern void sockopt_iphdrincl_swab_systoh (struct ip *iph);
 
+extern int sockopt_tcp_rtt (int);
 extern int sockopt_tcp_signature(int sock, union sockunion *su,
                                  const char *password);
 #endif /*_ZEBRA_SOCKOPT_H */