mlbasemodel: fix warnings on Clang 16.0.6

Fix the following warnings on clang 16.0.6:

../../../../modules/gui/qt/medialibrary/mlbasemodel.cpp:60:10: warning: 'validateCache' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
    void validateCache() const
         ^
../../../../modules/gui/qt/util/base_model_p.hpp:128:10: note: overridden virtual function is here
    void validateCache() const override;
     ^



../../../../modules/gui/qt/medialibrary/mlbasemodel.cpp:473:10: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
        [this, cb](quint64 taskId, Ctx& ctx)
         ^~~~~
../../../../modules/gui/qt/medialibrary/mlbasemodel.cpp:495:10: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
        [this, cb](quint64 taskId, Ctx& ctx)
         ^~~~~
../../../../modules/gui/qt/medialibrary/mlbasemodel.cpp:511:10: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
        [this, offset, limit, op = m_op]
         ^~~~~
../../../../modules/gui/qt/medialibrary/mlbasemodel.cpp:518:10: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
        [this, cb](quint64 taskId, Ctx& ctx) {
         ^~~~~
../../../../modules/gui/qt/medialibrary/mlbasemodel.cpp:592:10: warning: lambda capture 'this' is not used [-Wunused-lambda-capture]
        [this, cb](qint64 taskId, Ctx& ctx) {
         ^~~~~