Fix "reference to 'thread' is ambiguous"

Summary: This fixes github issue https://github.com/facebook/folly/issues/243

I don't understand why this is broken on osx, but the solution is to just `#include <thread>` in the header instead of trying to forward declare `std::thread`.

Reviewed By: @yfeldblum

Differential Revision: D2255026

folly/io/async/ScopedEventBaseThread.h

@@ -17,12 +17,9 @@
 #pragma once
 
 #include <memory>
+#include <thread>
 #include <folly/io/async/EventBase.h>
 
-namespace std {
-class thread;
-}
-
 namespace folly {
 
 /**