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

```
../argument.cpp:31:29: error: variable 'optionstr' is non-const and globally accessible, consider making it const [cppcoreguidelines-avoid-non-const-global-variables,-warnings-as-errors]
const char* ArgumentParser::optionstr = "p:?h";
                            ^
```

Change-Id: I8518db5525e0f67c8f263079a9389621094249bf
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
2 files changed