Make QtSensors use Q_DECL_OVERRIDE

This patch aims to make QtSensors uses Q_DECL_OVERRIDE uniformly across
the module

Task-number: QTBUG-27392
Change-Id: I5f814f93307159175f92e29a9990587c03cce96f
Reviewed-by: Thomas McGuire <thomas.mcguire@kdab.com>