sysutils/twmn: Update to g20190917, unbreak with clang 10

In file included from /usr/local/include/qt5/QtCore/qstring.h:49:
/usr/local/include/qt5/QtCore/qbytearray.h:545:22: error: definition of implicit copy constructor for 'QByteRef' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]

inline QByteRef &operator=(const QByteRef &c)
                 ^

/usr/local/include/qt5/QtCore/qbytearray.h:563:28: note: in implicit copy constructor for 'QByteRef' first required here
{ Q_ASSERT(i >= 0); return QByteRef(*this, i); }

^

PR: 244251