Return spelled default ctor in AlignedSysAllocator

Summary: [Folly] Return spelled default ctor in `AlignedSysAllocator` - it is necessary until gcc7.

Reviewed By: LeeHowes

Differential Revision: D19235864

fbshipit-source-id: fd0e9c33283e8b646aff56efb10d8e2a8c05e293

folly/Memory.h

@@ -477,6 +477,12 @@ class AlignedSysAllocator : private Align {
 
   using Align::Align;
 
+  // TODO: remove this ctor, which is is no longer required as of under gcc7
+  template <
+      typename S = Align,
+      std::enable_if_t<std::is_default_constructible<S>::value, int> = 0>
+  constexpr AlignedSysAllocator() noexcept(noexcept(Align())) : Align() {}
+
   template <typename U>
   constexpr explicit AlignedSysAllocator(
       AlignedSysAllocator<U, Align> const& other) noexcept