audio/musescore3: fix build on 15-CURRENT

Error message is

/wrkdirs/usr/ports/audio/musescore3/work/MuseScore-3.6.2/thirdparty/dtl/dtl/Diff.hpp:166:27: error: cannot assign to non-static data member within const member function 'enableTrivial'

166 |             this->trivial = true;

/wrkdirs/usr/ports/audio/musescore3/work/MuseScore-3.6.2/thirdparty/dtl/dtl/Diff.hpp:165:14: note: member function 'dtl::Diff::enableTrivial' is declared const here

165 |         void enableTrivial () const {

While enableTrivial() is never called -- it's a template, and
never instantiated -- clang is now more strict about would-be-ill-formed-
if-instantiated, it seems.