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.

drivers/unix/dir_access_unix.h

@@ -90,5 +90,6 @@ public:
 	~DirAccessUnix();
 };
 
-#endif //UNIX ENABLED
-#endif
+#endif // UNIX_ENABLED || LIBC_FILEIO_ENABLED
+
+#endif // DIR_ACCESS_UNIX_H