Fix PollIoBackend compile error

Summary:
Fix PollIoBackend compile error

(Note: this ignores all push blocking failures!)

Reviewed By: yfeldblum, kevin-vigor

Differential Revision: D20955224

fbshipit-source-id: a2c7a34d9129cb282d3652607e79dcee1ce1cc78

folly/experimental/io/PollIoBackend.h

@@ -154,7 +154,7 @@ class PollIoBackend : public EventBaseBackendBase {
     }
 
    private:
-    std::array<int, 2> fds_ = {-1, -1};
+    std::array<int, 2> fds_{{-1, -1}};
   };
 
   static FOLLY_ALWAYS_INLINE uint32_t getPollFlags(short events) {