audio/mp3plot: Fix build with Clang 6
/usr/include/c++/v1/memory:2097:9: error: cannot initialize a member subobject of type 'boost::shared_ptr<mp3plot::bitrate_plotter> *' with an rvalue of type 'int'
: __value_(_VSTD::forward<_Up>(__u)){};
^ ~~~~~~~~~~~~~~~~~~~~~~~~/usr/include/c++/v1/memory:2187:9: note: in instantiation of function template specialization 'std::1::compressed_pair_elem<boost::shared_ptr<mp3plot::bitrate_plotter> *, 0, false>::__compressed_pair_elem<int, void>' requested here
: _Base1(std::forward<_Tp>(__t)), _Base2() {}
^