COMP: Fix build errors with VTK6

Fix two minor compile errors in VTK6 conditional branch of
qSlicerSceneViewsModuleWidget, that were introduced in r23160.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23164 3bd1e089-480b-0410-8dfb-8563597acbee