[RF] Fix warning about getObservables member shadowing in RooProdPdf
[RF] Fix warning about getObservables member shadowing in RooProdPdf

This fixes warnings in the CentOS 7 build in the nightlies.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root