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/intern/bpy_app_ffmpeg.h

@@ -18,8 +18,7 @@
  * \ingroup pythonintern
  */
 
-#ifndef __BPY_APP_FFMPEG_H__
-#define __BPY_APP_FFMPEG_H__
+#pragma once
 
 #ifdef __cplusplus
 extern "C" {
@@ -30,5 +29,3 @@ PyObject *BPY_app_ffmpeg_struct(void);
 #ifdef __cplusplus
 }
 #endif
-
-#endif /* __BPY_APP_FFMPEG_H__ */