logind: fix "CanGraphical" attribute to return correct value

We should return seat_can_graphical() instead of seat_can_tty() for the
public dbus CanGraphical attribute. This used to work, but the
dbus -> sd-bus conversion introduced this regression.

src/login/logind-seat-dbus.c

@@ -103,7 +103,7 @@ static int property_get_can_graphical(
         assert(reply);
         assert(s);
 
-        return sd_bus_message_append(reply, "b", seat_can_tty(s));
+        return sd_bus_message_append(reply, "b", seat_can_graphical(s));
 }
 
 static int property_get_sessions(