Initialize LifoSem's padding to allow for its constexpr constructor.

Summary: MSVC correctly gives an error about the constexpr constructor not initializing all members.

Reviewed By: yfeldblum

Differential Revision: D2856806

fb-gh-sync-id: cef97639906dd3c39e3d3dc2ba939021e15edcb9

folly/LifoSem.h

@@ -323,7 +323,7 @@ struct LifoSemBase {
 
   /// Constructor
   constexpr explicit LifoSemBase(uint32_t initialValue = 0)
-    : head_(LifoSemHead::fresh(initialValue)) {}
+      : head_(LifoSemHead::fresh(initialValue)), padding_() {}
 
   LifoSemBase(LifoSemBase const&) = delete;
   LifoSemBase& operator=(LifoSemBase const&) = delete;