BoxDDP used extended initializer lists, feature of c++11

For more details see:

/devel/crocoddyl/include/crocoddyl/core/solvers/box-qp.hpp: In function ‘crocoddyl::BoxQPSolution crocoddyl::BoxQP(const MatrixXd&, const VectorXd&, const VectorXd&, const VectorXd&, const VectorXd&, double, int, double, double)’:
/devel/crocoddyl/include/crocoddyl/core/solvers/box-qp.hpp:48:12: warning: extended initializer lists only available with -std=c++11 or -std=gnu++11
     return {Hff_inv, x_init, free_idx, clamped_idx};
            ^
/devel/crocoddyl/include/crocoddyl/core/solvers/box-qp.hpp:66:14: warning: extended initializer lists only available with -std=c++11 or -std=gnu++11
       return {Hff_inv, x, free_idx, clamped_idx};
              ^
/devel/crocoddyl/include/crocoddyl/core/solvers/box-qp.hpp:141:10: warning: extended initializer lists only available with -std=c++11 or -std=gnu++11
   return {Hff_inv, x, free_idx, clamped_idx};

@wxmerkt -- could you handle this?