Fix PHP strict notice ::get_avatar()

```
[07-Nov-2013 21:12:56 UTC] PHP Strict Standards:  Declaration of HMA_Uploaded_Avatar_Option::get_avatar() should be compatible with HMA_SSO_Avatar_Option::get_avatar($size = NULL) in /srv/www/wpremote.dev/content/plugins-mu/hm-accounts/hm-accounts.sso.php on line 3
```