Refactor: Move BPY mathutils headers to proper C++ ones.

This module actually was confusing, since only a few headers were
effectively still C ones (using the `extern "C"` blocks)...

source/blender/python/mathutils/mathutils_kdtree.hh

@@ -0,0 +1,13 @@
+/* SPDX-FileCopyrightText: 2023 Blender Authors
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later */
+
+/** \file
+ * \ingroup mathutils
+ */
+
+#pragma once
+
+PyMODINIT_FUNC PyInit_mathutils_kdtree();
+
+extern PyTypeObject PyKDTree_Type;