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/generic/python_utildefines.h

@@ -20,8 +20,7 @@
  * \note light addition to Python.h, use py_capi_utils.h for larger features.
  */
 
-#ifndef __PYTHON_UTILDEFINES_H__
-#define __PYTHON_UTILDEFINES_H__
+#pragma once
 
 #ifdef __cplusplus
 extern "C" {
@@ -57,5 +56,3 @@ Py_LOCAL_INLINE(int) PyList_APPEND(PyObject *op, PyObject *v)
 #ifdef __cplusplus
 }
 #endif
-
-#endif /* __PYTHON_UTILDEFINES_H__ */