Code quality: Fix header guards consistency

Adds `header_guards.sh` bash script, used in CI to validate future
changes. Can be run locally to fix invalid header guards.

platform/macos/dir_access_macos.h

@@ -51,5 +51,6 @@ protected:
 	virtual bool is_hidden(const String &p_name);
 };
 
-#endif //UNIX ENABLED
-#endif
+#endif // UNIX ENABLED || LIBC_FILEIO_ENABLED
+
+#endif // DIR_ACCESS_MACOS_H