ntp_io.c:process_routing_msgs on CentOS 6.8 -- errors with rtm

[Dan Drown]

Quoting Hal Murray <hmurray at megapathdsl.net>:
> It looks like you are missing /usr/include/linux/rtnetlink.h
> Do you have kernel-headers installed?
>
> It builds on FreeBSD without rtnetlink.h so there is probably something more
> complicated going on.
>
> Does your net/route.h define rt_msghdr?  That's where it comes from on
> FreeBSD.  Or anyplace else in /usr/include/?

There's a pull request open for this:
https://gitlab.com/NTPsec/ntpsec/merge_requests/31

"CentOS 6 requires sys/socket.h before including linux/rtnetlink.h or  
linux/if_addr.h"