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_bvhtree.h

@@ -18,8 +18,7 @@
  * \ingroup mathutils
  */
 
-#ifndef __MATHUTILS_BVHTREE_H__
-#define __MATHUTILS_BVHTREE_H__
+#pragma once
 
 PyMODINIT_FUNC PyInit_mathutils_bvhtree(void);
 
@@ -27,5 +26,3 @@ extern PyTypeObject PyBVHTree_Type;
 
 #define PyBVHTree_Check(v) PyObject_TypeCheck((v), &PyBVHTree_Type)
 #define PyBVHTree_CheckExact(v) (Py_TYPE(v) == &PyBVHTree_Type)
-
-#endif /* __MATHUTILS_BVHTREE_H__ */