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/functions/FN_multi_function_network_optimization.hh

@@ -14,8 +14,7 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
-#ifndef __FN_MULTI_FUNCTION_NETWORK_OPTIMIZATION_HH__
-#define __FN_MULTI_FUNCTION_NETWORK_OPTIMIZATION_HH__
+#pragma once
 
 #include "FN_multi_function_network.hh"
 
@@ -28,5 +27,3 @@ void constant_folding(MFNetwork &network, ResourceCollector &resources);
 void common_subnetwork_elimination(MFNetwork &network);
 
 }  // namespace blender::fn::mf_network_optimization
-
-#endif /* __FN_MULTI_FUNCTION_NETWORK_OPTIMIZATION_HH__ */