cmake-gui: Clarify signature of QCheckBox::checkStateChanged callbacks

In commit cbf0d3da52 (cmake-gui: Port away from deprecated API in Qt >=
6.7, 2025-02-21, v4.1.0-rc1~755^2~4) we assumed that `Qt::CheckState` is
implemented as an `int`.  Spell out the type explicitly for the Qt 6.7+
callback signatures.