sys/socket.h: add SO_BINDTOIFINDEX from linux v5.1

SO_BINDTOIFINDEX behaves similar to SO_BINDTODEVICE, but takes a
network interface index as argument, rather than the network
interface name. see

  linux commit f5dd3d0c9638a9d9a02b5964c4ad636f06cf7e2c
  net: introduce SO_BINDTOIFINDEX sockopt

include/sys/socket.h

@@ -237,6 +237,7 @@ struct linger {
 #define SO_ZEROCOPY             60
 #define SO_TXTIME               61
 #define SCM_TXTIME              SO_TXTIME
+#define SO_BINDTOIFINDEX        62
 
 #ifndef SOL_SOCKET
 #define SOL_SOCKET      1