FIX logout message security error from idp

" The request cannot be fulfilled because the message received does not
meet the security requirements of the login service. " I think this
means we need to sign our logout messages.

I think this only happens because its trying to do a singlelogout.
While I'd rather just disable singlelogout, there doesn't seem to be a
simple toggle to do this. So I'm trying to get slo working instead.

docker/simplesamlphp/sp/config/authsources.php

@@ -75,6 +75,9 @@
             'urn:oid:x.x.x.x',
         ],
         */
+
+        // IDP message security error on singlelogout without this
+        'sign.logout' => true,
     ],