add missing use statement in class DecrementOptions

[deminy]

In v4.1.6, the following code piece will fail when executed:

<?php

// ...
$options = new Couchbase\DecrementOptions();
$options->initial(1);
$options->expiry((new DateTimeImmutable())->modify('+5 seconds'));

$collection->binary()->decrement($key, $options);
?>

The error message looks like the following:

Warning: Object of class DateTimeImmutable could not be converted to int in vendor/couchbase/couchbase/Couchbase/DecrementOptions.php on line 123

[cb-sdk-robot]

Can one of the admins verify this patch?