Added XID to logout URL

The link generated by ```{exp:freemember:logout_url}``` in EE 2.7+ fails because it does not include an XID. An XID is usually only required for POST submissions. However, since the ```act_logout``` method utilizes ```member_logout``` from the core EE member class, the ```secure_forms_check``` filter applies in this GET context. Not wanting to hack the core, I've added the XID hash to the logout URL that ```logout_url``` generates to bypass this issue.