Return spelled default ctor in AlignedSysAllocator
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

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly