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/mathutils/mathutils_Vector.h

@@ -18,8 +18,7 @@
  * \ingroup pymathutils
  */
 
-#ifndef __MATHUTILS_VECTOR_H__
-#define __MATHUTILS_VECTOR_H__
+#pragma once
 
 extern PyTypeObject vector_Type;
 
@@ -48,5 +47,3 @@ PyObject *Vector_CreatePyObject_alloc(float *vec,
                                       const int size,
                                       PyTypeObject *base_type) ATTR_WARN_UNUSED_RESULT
     ATTR_NONNULL(1);
-
-#endif /* __MATHUTILS_VECTOR_H__ */