Build fix for gcc

Summary: This worked in clang but not in gcc. Quick fix

Differential Revision: D7800576

fbshipit-source-id: fcd04c0779f8a176043f3dede61289830a1ef607

folly/stats/detail/BufferedStat-defs.h

@@ -98,7 +98,7 @@ void BufferedDigest<DigestT, ClockT>::onNewDigest(
     typename ClockT::time_point /*newExpiry*/,
     typename ClockT::time_point /*oldExpiry*/,
     const std::unique_lock<SharedMutex>& /*g*/) {
-  std::array<DigestT, 2> a{digest_, std::move(digest)};
+  std::array<DigestT, 2> a{{digest_, std::move(digest)}};
   digest_ = DigestT::merge(a);
 }