Draft: mingw: Add missing libopus-0.dll. Fixes #690.
As it says in the title. fluid~ has been broken (again) on Windows since 2.11.0. This MR should fix it, but my initial local build gave me trouble in
make check, so there might still be a few more Windows issues with libfluidsynth which are now uncovered by the fix. Therefore I'm keeping this as WIP for now, until I get these sorted out.