Mitigation for spurious warning in Octave

Because of a bug, Octave wrongly complains that matlab/+pac/+bgp/{get,set}.m
shadow built-in functions (see https://savannah.gnu.org/bugs/?46849).

We disable the warning as early as we can after running the dynare command.
However the warning will nevertheless appear two times in a given Octave
session (the first time when the addpath to Dynare is done, the second time
when the dynare command is run).