Add TCP_MD5SIG_FLAG_IFINDEX from Linux 5.6 to netinet/tcp.h.

This patch adds the TCP_MD5SIG_FLAG_IFINDEX constant from Linux 5.6 to
sysdeps/gnu/netinet/tcp.h and updates struct tcp_md5sig accordingly to
contain the device index.

Reviewed-by: Florian Weimer <fweimer@redhat.com>