[wpilib] PIDController: Recompute the error in AtSetpoint() (#2822)

This makes AtSetpoint() return false after the setpoint is changed with
SetSetpoint().

Closes #2821.

Co-authored-by: Prateek Machiraju <prateek.machiraju@gmail.com>

wpilibc/src/main/native/include/frc/controller/PIDController.h

@@ -238,6 +238,7 @@ class PIDController : public frc::Sendable,
   double m_velocityTolerance = std::numeric_limits<double>::infinity();
 
   double m_setpoint = 0;
+  double m_measurement = 0;
 };
 
 }  // namespace frc2