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_geometry.h

@@ -14,13 +14,10 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
-#ifndef __MATHUTILS_GEOMETRY_H__
-#define __MATHUTILS_GEOMETRY_H__
+#pragma once
 
 /** \file
  * \ingroup pymathutils
  */
 
 PyMODINIT_FUNC PyInit_mathutils_geometry(void);
-
-#endif /* __MATHUTILS_GEOMETRY_H__ */