Fix `-Wnested-anon-types`

Reviewed By: igorsugak

Differential Revision: D32415926

fbshipit-source-id: b62ac87215800eed8e6cf8acf92f0518b9a56da5

folly/fibers/Baton.h

@@ -250,12 +250,14 @@ class Baton {
   static constexpr intptr_t TIMEOUT = -2;
   static constexpr intptr_t THREAD_WAITING = -3;
 
-  union {
-    std::atomic<intptr_t> waiter_;
-    struct {
+  struct _futex_wrapper {
     folly::detail::Futex<> futex{};
     int32_t _unused_packing;
-    } futex_;
+  };
+
+  union {
+    std::atomic<intptr_t> waiter_;
+    struct _futex_wrapper futex_;
   };
 };