package/paho-mqtt-c: security bump to version 1.3.12

Contains a fix for a possible race condition that can lead to use after
free:
https://github.com/eclipse/paho.mqtt.c/commit/911488a9f0fdad31c85ec86712be52587afdbbb8

https://github.com/eclipse/paho.mqtt.c/milestone/19?closed=1

https://github.com/eclipse/paho.mqtt.c/releases/tag/v1.3.12



Signed-off-by: Daniel Lang <d.lang@abatec.at>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>