[platforms/hwcomposer] Fix build failure after D23495

    In file included from /build/src/kwin-5.16.90/plugins/platforms/hwcomposer/hwcomposer_backend.cpp:21:
    /build/src/kwin-5.16.90/plugins/platforms/hwcomposer/hwcomposer_backend.h:58:11: error:
    'QSize KWin::HwcomposerOutput::pixelSize() const' marked 'override', but does not override
       58 |     QSize pixelSize() const override;
          |           ^~~~~~~~~

Fixes dbb2cede



Reviewed-by: Bhushan Shah <bshah@kde.org>