cut dep from Expected to Optional

Summary: Expected no longer uses Optional, so remove the `#include`.

Reviewed By: nlutsenko

Differential Revision: D56065311

fbshipit-source-id: fa0d8473e5b7ca749f40f68a9ee76b7be121d7b7

Author: yfeldblum

cachelib/common/EventInterface.h

@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <folly/Optional.h>
 #include <folly/logging/xlog.h>
 
 namespace facebook {