qt: mlfolderseditor: fix capture-by-copy parameters
Fixes the following warning:
../modules/gui/qt/widgets/native/mlfolderseditor.cpp:144:55: warning: explicit capture of 'this' with a capture default of '=' is a C++20 extension [-Wc++20-extensions]
144 | connect( pb, &QPushButton::clicked, this, [=, this]()
| ^