netinet/tcp.h: add TCP_NLA_SRTT from linux v5.0

smoothed RTT for SCM_TIMESTAMPING_OPT_STATS control messages.
added in linux commit e8bd8fca6773ef49390269bd467bf940a0841ccf

include/netinet/tcp.h

@@ -76,6 +76,7 @@ enum {
 	TCP_NLA_BYTES_RETRANS,
 	TCP_NLA_DSACK_DUPS,
 	TCP_NLA_REORD_SEEN,
+	TCP_NLA_SRTT,
 };
 
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)