Guard the forward declaration of fibers::Baton

Summary: [Folly] Guard the forward declaration of `fibers::Baton` by the same condition which determines whether `fibers::Baton` is ever used in futures.

Reviewed By: lewissbaker

Differential Revision: D17962552

fbshipit-source-id: 2494026999cd170c36adec91108732254212abf1

folly/futures/Future.h

@@ -108,10 +108,14 @@ class SemiFuture;
 template <class T>
 class FutureSplitter;
 
+#if FOLLY_FUTURE_USING_FIBER
+
 namespace fibers {
 class Baton;
 }
 
+#endif
+
 namespace futures {
 namespace detail {
 template <class T>