QSplineSeries: suppress warning from initializeGraphics

QLineSeriesPrivate implements initializeGraphics, and Axivion warns
about the QSplineSeriesPrivate override skipping that immediate base
class. However, the base class implementation allocates a LineChartItem,
which we evidently (as we didn't so far) don't want to happen. Skipping
of the base class is intentional, so suppress the warning.

Task-number: QTBUG-146050
Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV545
Pick-to: 6.12
Change-Id: Ic6e92e2868618d976f77c6bf31263f0c0e45b800
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>