QmlWaveformOverview: fix float conversion warning with Qt6

[Be-ing]

[184/695] Building CXX object CMakeFiles/mixxx-lib.dir/src/qml/qmlwaveformoverview.cpp.o
../src/qml/qmlwaveformoverview.cpp: In member function ‘QColor mixxx::qml::QmlWaveformOverview::getRgbPenColor(ConstWaveformPointer, int) const’:
../src/qml/qmlwaveformoverview.cpp:258:27: warning: conversion from ‘qreal’ {aka ‘double’} to ‘float’ may change value [-Wfloat-conversion]
  258 |         color.setRgbF(red / max, green / max, blue / max);
      |                       ~~~~^~~~~
../src/qml/qmlwaveformoverview.cpp:258:40: warning: conversion from ‘qreal’ {aka ‘double’} to ‘float’ may change value [-Wfloat-conversion]
  258 |         color.setRgbF(red / max, green / max, blue / max);
      |                                  ~~~~~~^~~~~
../src/qml/qmlwaveformoverview.cpp:258:52: warning: conversion from ‘qreal’ {aka ‘double’} to ‘float’ may change value [-Wfloat-conversion]
  258 |         color.setRgbF(red / max, green / max, blue / max);
      |                                               ~~~~~^~~~~