libsForQt5.fcitx5-qt: fix reference to qt6.qtbase

  cmakeFlags = [

    # adding qt6 to buildInputs would result in error: detected mismatched Qt dependencies

    "-DCMAKE_PREFIX_PATH=${qt6.qtbase.dev}"

    "-DCMAKE_PREFIX_PATH=${qt6.qtbase}"

    "-DENABLE_QT4=0"

    "-DENABLE_QT6=1"

  ];