add MSG_FASTOPEN sendmsg/sendto flag to socket.h

This was new in linux 3.5 in commit cf60af03ca4e71134206809ea892e49b92a88896,
needed for tcp fastopen feature (sending data in TCP SYN packet).

include/sys/socket.h

@@ -255,6 +255,7 @@ struct linger
 #define MSG_NOSIGNAL  0x4000
 #define MSG_MORE      0x8000
 #define MSG_WAITFORONE 0x10000
+#define MSG_FASTOPEN  0x20000000
 #define MSG_CMSG_CLOEXEC 0x40000000
 
 #define __CMSG_LEN(cmsg) (((cmsg)->cmsg_len + sizeof(long) - 1) & ~(long)(sizeof(long) - 1))