BaseTrackTableModel: fix float conversion warning

[Be-ing]

[311/695] Building CXX object
CMakeFiles/mixxx-lib.dir/src/library/basetracktablemodel.cpp.o
../src/library/basetracktablemodel.cpp: In member function ‘virtual
QVariant BaseTrackTableModel::data(const QModelIndex&, int) const’:
../src/library/basetracktablemodel.cpp:423:27: warning: conversion from
‘double’ to ‘float’ may change value [-Wfloat-conversion]
  423 |         bgColor.setAlphaF(m_backgroundColorOpacity);
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~

[Holzhaus]

Thanks.