logind: fix sysfs change trigger code

We can't create files in sysfs, hence don't bother. Also if we ignore
the return value, do so explicitly by casting to void.

src/login/logind-dbus.c

@@ -1282,7 +1282,7 @@ static int trigger_device(Manager *m, struct udev_device *d) {
                 if (!t)
                         return -ENOMEM;
 
-                write_string_file(t, "change", WRITE_STRING_FILE_CREATE);
+                (void) write_string_file(t, "change", 0);
         }
 
         return 0;