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_shader.h

@@ -18,8 +18,7 @@
  * \ingroup bpygpu
  */
 
-#ifndef __GPU_PY_SHADER_H__
-#define __GPU_PY_SHADER_H__
+#pragma once
 
 extern PyTypeObject BPyGPUShader_Type;
 
@@ -32,5 +31,3 @@ typedef struct BPyGPUShader {
 
 PyObject *BPyGPUShader_CreatePyObject(struct GPUShader *shader, bool is_builtin);
 PyObject *BPyInit_gpu_shader(void);
-
-#endif /* __GPU_PY_SHADER_H__ */