Clean up top-level Eigen headers

libeigen/eigen!2252

Co-authored-by: Rasmus Munk Larsen <rmlarsen@gmail.com>

Eigen/Dense

@@ -1,3 +1,13 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef EIGEN_DENSE_MODULE_H
+#define EIGEN_DENSE_MODULE_H
+
 #include "Core"
 #include "LU"
 #include "Cholesky"
@@ -5,3 +15,5 @@
 #include "SVD"
 #include "Geometry"
 #include "Eigenvalues"
+
+#endif  // EIGEN_DENSE_MODULE_H