[wpimath] Use external Eigen headers only (#3564)

Internal headers are no longer allowed as of
https://gitlab.com/libeigen/eigen/-/merge_requests/631. Based on
benchmarking I conducted in that thread, there doesn't seem to be a
performance penalty for including the full headers anymore.

wpimath/src/main/native/include/frc/system/Discretization.h

@@ -5,9 +5,8 @@
 #pragma once
 
 #include "Eigen/Core"
-#include "Eigen/src/LU/PartialPivLU.h"
 #include "units/time.h"
-#include "unsupported/Eigen/src/MatrixFunctions/MatrixExponential.h"
+#include "unsupported/Eigen/MatrixFunctions"
 
 namespace frc {