sys/socket.h: add SO_TXTIME from linux v4.19

added in linux commit 80b14dee2bea128928537d61c333f24cb8cbb62f

include/sys/socket.h

@@ -235,6 +235,8 @@ struct linger {
 #define SCM_TIMESTAMPING_PKTINFO 58
 #define SO_PEERGROUPS           59
 #define SO_ZEROCOPY             60
+#define SO_TXTIME               61
+#define SCM_TXTIME              SO_TXTIME
 
 #ifndef SOL_SOCKET
 #define SOL_SOCKET      1