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/blenlib/BLI_math_base_safe.h

@@ -14,8 +14,7 @@
  * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
  */
 
-#ifndef __BLI_MATH_BASE_SAFE_H__
-#define __BLI_MATH_BASE_SAFE_H__
+#pragma once
 
 /** \file
  * \ingroup bli
@@ -46,5 +45,3 @@ MINLINE float safe_powf(float base, float exponent);
 #if BLI_MATH_DO_INLINE
 #  include "intern/math_base_safe_inline.c"
 #endif
-
-#endif /* __BLI_MATH_BASE_SAFE_H__ */