QCategoryAxis: suppress Axivion warning in initializeGraphics

QValueAxisPrivate::initializeGraphics allocates objects, which we
evidently didn't want or need so far, so suppress the warning about us
skipping the direct base class in the virtual override.

Axivion-Id: qt_Add-ons_qtcharts_dev_linux:SV35
Task-number: QTBUG-146050
Pick-to: 6.12
Change-Id: If835886892b64c71495b12ba0811d44099625397
Reviewed-by: Tomi Korpipää <tomi.korpipaa@qt.io>
Reviewed-by: Sami Varanka <sami.varanka@qt.io>