argument: Fix bugprone-reserved-identifier

```
/home/andrew/src/openbmc/phosphor-led-sysfs/build/../argument.hpp:2:9: error: declaration uses identifier '__ARGUMENT_H', which is a reserved identifier [bugprone-reserved-identifier,cert-dcl37-c,cert-dcl51-cpp,-warnings-as-errors]
        ^~~~~~~~~~~~
        ARGUMENT_H
```

Change-Id: Ib68d3f29b6917872e5d9d5e76a8a5c79c96ae2a2
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/argument.hpp b/argument.hpp
index 8240eb7..332ecce 100644
--- a/argument.hpp
+++ b/argument.hpp
@@ -1,5 +1,5 @@
-#ifndef __ARGUMENT_H
-#define __ARGUMENT_H
+#ifndef ARGUMENT_H
+#define ARGUMENT_H
 #include <getopt.h>
 
 #include <map>