[C++] Fix required mosquitto version in CMakeLists.txt
[C++] Fix required mosquitto version in CMakeLists.txt

* Set required version of mosquitto back to 1.6.8.
* The update to version 1.6.8 in
  71deebf80794c36bfa3cabc5ebc0f0307159e346 got lost during git
  rebase in 33792269e708871bb067a40b8fc7027974702231.

bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr,bmwcarit/joynr