PHPLIB-1400: Add missing use statement (#1235)

Fixes an issue from a5a39ec

Author: jmikola

tests/UnifiedSpecTests/CollectionData.php

@@ -6,6 +6,7 @@
 use MongoDB\Client;
 use MongoDB\Driver\ReadConcern;
 use MongoDB\Driver\ReadPreference;
+use MongoDB\Driver\Session;
 use MongoDB\Driver\WriteConcern;
 use MongoDB\Tests\UnifiedSpecTests\Constraint\Matches;
 use MultipleIterator;