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
I removed bf_blenkernel from `nodes/CMakeLists.txt` again (added it yesterday),
because now this was causing me unresolved symbol errors... Without it, cmake
seems to link the libraries bf_simulation, bf_blenkernel and bf_nodes in the right
order. Not sure if that is just luck or if it is guaranteed.
It was possible to fix the issue by using cmakes `LINK_INTERFACE_MULTIPLICITY`,
but that is probably bad style.
