argument: Fix cppcoreguidelines-avoid-non-const-global-variables

```
../argument.hpp:52:31: error: variable 'optionstr' is non-const and globally accessible, consider making it const [cppcoreguidelines-avoid-non-const-global-variables,-warnings-as-errors]
    static inline const char* optionstr = "p:?h";
                              ^
```

Change-Id: I2c22a9f2c5cd9dbe0ce23928663dd10679f873c4
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
1 file changed