TrkVolumes: Fix for cppcheck
The new cpp_check
nightlies reported the following warning in TrkVolumes
/build/atnight/localbuilds/nightlies/Athena/main--cppcheck/athena/Tracking/TrkDetDescr/TrkVolumes/src/SimplePolygonBrepVolumeBounds.cxx:149:37: warning: Member variable 'SimplePolygonBrepVolumeBounds::m_ordering' is not assigned a value in 'SimplePolygonBrepVolumeBounds::operator='. [operatorEqVarError]
This MR should fix this.