Code Style: use "#pragma once" in source directory

This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.

This patch has been generated by P1561 followed by `make format`.

Differential Revision: https://developer.blender.org/D8466

source/blender/python/mathutils/mathutils_Matrix.h

@@ -18,8 +18,7 @@
  * \ingroup pymathutils
  */
 
-#ifndef __MATHUTILS_MATRIX_H__
-#define __MATHUTILS_MATRIX_H__
+#pragma once
 
 extern PyTypeObject matrix_Type;
 extern PyTypeObject matrix_access_Type;
@@ -96,5 +95,3 @@ extern struct Mathutils_Callback mathutils_matrix_col_cb;
 extern struct Mathutils_Callback mathutils_matrix_translation_cb;
 
 void matrix_as_3x3(float mat[3][3], MatrixObject *self);
-
-#endif /* __MATHUTILS_MATRIX_H__ */