Fix for building with icc.

Summary: Closes https://github.com/facebook/folly/pull/649

Reviewed By: Orvid

Differential Revision: D5535534

Pulled By: yfeldblum

fbshipit-source-id: 641186d6171479755bd9471a484b8b3c2ab62418

folly/detail/Futex.h

@@ -46,7 +46,7 @@ enum class FutexResult {
 template <template <typename> class Atom = std::atomic>
 struct Futex : Atom<uint32_t>, boost::noncopyable {
 
-  explicit Futex(uint32_t init = 0) : Atom<uint32_t>(init) {}
+  explicit constexpr Futex(uint32_t init = 0) : Atom<uint32_t>(init) {}
 
   /** Puts the thread to sleep if this->load() == expected.  Returns true when
    *  it is returning because it has consumed a wake() event, false for any