pam-authenticate: fix clang-tidy warning

```
../include/pam_authenticate.hpp:11:75: error: 'msg' is an unsafe pointer used for buffer access [-Werror,-Wunsafe-buffer-usage]
inline int pamFunctionConversation(int numMsg, const struct pam_message** msg,
```

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Ic0e6d63b01eea78cac54407246363177cb208f8b