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.h

@@ -14,8 +14,7 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
-#ifndef __MATHUTILS_H__
-#define __MATHUTILS_H__
+#pragma once
 
 /** \file
  * \ingroup pymathutils
@@ -195,5 +194,3 @@ int column_vector_multiplication(float rvec[4], VectorObject *vec, MatrixObject
 /* dynstr as python string utility functions */
 PyObject *mathutils_dynstr_to_py(struct DynStr *ds);
 #endif
-
-#endif /* __MATHUTILS_H__ */