planning: fix compile warning.

modules/planning/math/smoothing_spline/spline_1d_constraint.cc

@@ -420,7 +420,8 @@ bool Spline1dConstraint::AddDerivativeSmoothConstraint() {
     return false;
   }
 
-  const uint32_t n_constraint = (x_knots_.size() - 2) * 2;
+  const uint32_t n_constraint =
+      (static_cast<uint32_t>(x_knots_.size()) - 2) * 2;
   const uint32_t num_params = spline_order_ + 1;
   Eigen::MatrixXd equality_constraint =
       Eigen::MatrixXd::Zero(n_constraint, (x_knots_.size() - 1) * num_params);
@@ -456,7 +457,8 @@ bool Spline1dConstraint::AddSecondDerivativeSmoothConstraint() {
     return false;
   }
 
-  const uint32_t n_constraint = (x_knots_.size() - 2) * 3;
+  const uint32_t n_constraint =
+      (static_cast<uint32_t>(x_knots_.size()) - 2) * 3;
   const uint32_t num_params = spline_order_ + 1;
   Eigen::MatrixXd equality_constraint =
       Eigen::MatrixXd::Zero(n_constraint, (x_knots_.size() - 1) * num_params);
@@ -505,7 +507,8 @@ bool Spline1dConstraint::AddThirdDerivativeSmoothConstraint() {
     return false;
   }
 
-  const uint32_t n_constraint = (x_knots_.size() - 2) * 4;
+  const uint32_t n_constraint =
+      (static_cast<uint32_t>(x_knots_.size()) - 2) * 4;
   const uint32_t num_params = spline_order_ + 1;
   Eigen::MatrixXd equality_constraint =
       Eigen::MatrixXd::Zero(n_constraint, (x_knots_.size() - 1) * num_params);