Silence warning for unused variable rsp in class unsubscribe_response

[tnorth]

Trivial change to avoid the warning for unused variable rsp, problematic when using -Werror in a project including server_response.h as header.

[fpagliughi]

Hey, thanks for the PR! I'm just getting back to a new release of this project after a while.

Unfortunately, as this is an Eclipse project, I can't merge anything without a signed ECA. And I don't think it would be woth your time with something so trivial. I can fix it.

But, with C++, my preferred way would be to comment out the parameter rather than the C trick of using (void) inside the function body, so:

unsubscribe_response(MQTTAsync_successData* /*rsp*/) {}

But more importantly, you made me realize that this project doesn't have build warnings turned on at all. (Sorry the CMake build was done years ago by someone else). So I'm fixings that as well... and it uncovered a bug in the synchronous client!

So, thanks again. This was helpful.