Summary:
D1261546 introduced regression in `sizeof(ConcurrentSkipList)`.
In case of `NodeAlloc = folly::SysAlloc`, size of an empty `NodeAlloc` struct
is 1 byte, due to the alignment requirements of the next field, it got
translated into wasting 8 bytes.

Test Plan: fbconfig -r folly/test && fbmake opt -j32

@override-unit-failures

Reviewed By: lucian@fb.com

FB internal diff: D1264730