multimedia/bombono: unbreak with gcc6 or later

src/mgui/dvdimport.cpp: In function 'void DVD::OnPreparePage(DVD::ImportData&)':
src/mgui/dvdimport.cpp:214:82: error: ambiguous overload for 'operator<<' (operand types are 'str::stream' and 'const char [3]')

std::string desc = (str::stream(Mpeg::SecToHMS(vob.tmLen, true)) <<  ", "
                         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

src/mgui/dvdimport.cpp: In function 'DVD::ReaderPtr DVD::OpenDVD(const string&, DVD::ImportData&)':
src/mgui/dvdimport.cpp:278:41: error: cannot convert 'DVD::ReaderPtr {aka ptr::shared<DVD::Reader>}' to 'bool' for argument '2' to 'void DVD::SetCurPageComplete(Gtk::Assistant&, bool)'

SetCurPageComplete(id.ast, id.reader);
                                    ^