Don't name a member small

Don't name a member small

Summary:
Because, if you aren't compiling with `WIN32_LEAN_AND_MEAN`, including `WinSock2.h` will result in `small` being `#define`'d as `char`. The best way to work around the issue is to simply change the name of the member to something else.
It would also have been possible to solve the problem by adding a section to `folly/portability/Windows.h`, but that would have required including that header in `folly/Function.h`, which there really is no reason to do.

Reviewed By: yfeldblum

Differential Revision: D3666417

fbshipit-source-id: 21c0363a4b77bb01e4ec20b2fc625c40729879c3