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/idprop_py_api.h

@@ -18,8 +18,7 @@
  * \ingroup pygen
  */
 
-#ifndef __IDPROP_PY_API_H__
-#define __IDPROP_PY_API_H__
+#pragma once
 
 struct BPy_IDGroup_Iter;
 struct ID;
@@ -68,5 +67,3 @@ PyObject *BPyInit_idprop(void);
 
 #define IDPROP_ITER_KEYS 0
 #define IDPROP_ITER_ITEMS 1
-
-#endif /* __IDPROP_PY_API_H__ */