[wpimath] Move C++ filters into filter folder to match Java (#3417)

wpilibcExamples/src/main/cpp/examples/DifferentialDrivePoseEstimator/cpp/Robot.cpp

@@ -2,9 +2,9 @@
 // Open Source Software; you can modify and/or share it under the terms of
 // the WPILib BSD license file in the root directory of this project.
 
-#include <frc/SlewRateLimiter.h>
 #include <frc/TimedRobot.h>
 #include <frc/XboxController.h>
+#include <frc/filter/SlewRateLimiter.h>
 
 #include "Drivetrain.h"