Add missing include to fix OSS build
Add missing include to fix OSS build

Summary: Without it there are a ton of errors due `ProxyConfig` not having been defined by the time it was needed.

Reviewed By: jmswen

Differential Revision: D4142072

fbshipit-source-id: 6376ee530e47c4152cd1c8fefdccde83e39c6ffa

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter