add pacing rate information to the tcp_info struct in tcp.h

used by monitoring applications such as ss from iproute2
introduced in linux 3.15 commit 977cb0ecf82eb6d15562573c31edebf90db35163

include/netinet/tcp.h

@@ -162,6 +162,8 @@ struct tcp_info
 	uint32_t tcpi_rcv_rtt;
 	uint32_t tcpi_rcv_space;
 	uint32_t tcpi_total_retrans;
+	uint64_t tcpi_pacing_rate;
+	uint64_t tcpi_max_pacing_rate;
 };
 
 #define TCP_MD5SIG_MAXKEYLEN    80