install python3-libgpiod on iot images

[jadonk]

Needed for rcpy

[RobertCNelson]

fixed with: https://github.com/RobertCNelson/omap-image-builder/commit/f0736fda14c4bf3967bad1914cf3c84c2f74bd00

also out starting with today:

https://rcn-ee.net/rootfs/bb.org/testing/2019-07-21/buster-iot/

Regards,