Add `#include <functional>` to `example.cpp`

[Adrian-Samoticha]

Considering that the example.cpp file makes use of std::function, shouldn’t it include <functional>, then?

Compiling without the aforementioned include state throws the following compiler error on my end:

main.cpp: In function ‘void show_impl(const msgpack11::MsgPack&, int)’:
main.cpp:119:67: error: ‘function’ is not a member of ‘std’
  119 |     using func_pair = std::tuple< bool (MsgPack::*)() const, std::function< void(MsgPack const&, int) > >;
      |                                                                   ^~~~~~~~