Fix common struc missmatch
Fix mismatch between common/steering/ (in steering.inc) and struct steering_ (in xfitter_cpp.h) from extra vars introduced by Valerio on 25 aug 2016, 4b4301b6. Add a simple check to detect these bugs in future. The bug seems to be harmless for the FrozenFrog release ( variables under vIPDFSET are not used in c++ code)