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/gpu/gpu_py_select.h

@@ -18,9 +18,6 @@
  * \ingroup bpygpu
  */
 
-#ifndef __GPU_PY_SELECT_H__
-#define __GPU_PY_SELECT_H__
+#pragma once
 
 PyObject *BPyInit_gpu_select(void);
-
-#endif /* __GPU_PY_SELECT_H__ */