fix OutputMagneticFieldDDToDDL (wasn't compiled by default)

Let bin/stubs/OutputMagneticFieldDDToDDL.cc be compiled by default (code was there, but wasn't included in the BuildFile). Also, update so that it compiles in 72X (was not updated since 71X).

This is a very minor fix that does not affect any functionality except this small module that is used to dump the MF geometry to a DB.