Fix using Atom with MSVC

Summary: Closes https://github.com/facebook/folly/pull/770

Reviewed By: nbronson

Differential Revision: D6991705

Pulled By: yfeldblum

fbshipit-source-id: 9f6ae0e09b436993ee998fe6c5d347121c229489

folly/detail/Futex.h

@@ -44,7 +44,9 @@ enum class FutexResult {
  */
 template <template <typename> class Atom = std::atomic>
 struct Futex : Atom<uint32_t> {
-  using Atom<uint32_t>::Atom;
+  Futex() : Atom<uint32_t>() {}
+
+  explicit constexpr Futex(uint32_t init) : 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