New version of Coap Simple Library breaks iotsa build

[jackjansen]

The newest version of Coap Simple Library breaks iotsa builds, because the callback pointer is a C function pointer again (as opposed to a std::function), to fix an issue with using Coap Simple Library with older C compilers.

Created https://github.com/hirotakaster/CoAP-simple-library/issues/22 and worked around the issue for iotsa by hard-coding Coap Simple [email protected].

Issue is mainly a reminder to check when/if this can be relieved.