minus_logged_prior_density.m: fix syntax error with output arguments

It’s not possible to use tildes for declaration of output arguments (otherwise
these arguments will be uninitialized if requested).

Detected by MATLAB Code Analyzer app.

matlab/estimation/minus_logged_prior_density.m

-function [fval, info, exitflag, ~, ~] = minus_logged_prior_density(xparams, Prior, options_, M_, estim_params_, oo_)
+function [fval, info, exitflag, fake1, fake2] = minus_logged_prior_density(xparams, Prior, options_, M_, estim_params_, oo_)
 
 % Evaluates minus the logged prior density.
  %
@@ -33,6 +33,8 @@ function [fval, info, exitflag, ~, ~] = minus_logged_prior_density(xparams, Prio
 
 exitflag = true;
 info = zeros(4,1);
+fake1 = [];
+fake2 = [];
 
 %------------------------------------------------------------------------------
 % 1. Get the structural parameters & define penalties