output/ipfix: flowDirection value placed in wrong field

[FestplattenSchnitzel]

https://github.com/CESNET/ipfixprobe/blob/480f2084a5aa6c86f0bd82b91549ba4aca678be9/init/link0.conf.example#L73

The configuration option output_plugin.ipfix.exporter.dir matches the semantic of the IPFIX flowDirection (61) field.

https://github.com/CESNET/ipfixprobe/blob/480f2084a5aa6c86f0bd82b91549ba4aca678be9/include/ipfixprobe/ipfix-elements.hpp#L88

However, the value is placed in the ingressInterface (10) field.

This leads to problems with flow collectors (e.g. Akvorado) as the value 0 is not allowed in this field, also see RFC 2863.