[cpp] I should not have to #include <proton/work_queue.hpp> to pass C++11 lambdas to schedule()

#include <proton/work_queue.hpp>
// ...
c.schedule(proton::duration::IMMEDIATE, [&c]() { c.stop(); });

Without the include, I get error: no matching function for call to ‘proton::container::schedule(const proton::duration&, main(int, char**)::<lambda()>)’.

There was discussion about this in comments on ENTMQCL-601.