[libcxx] Add missing <cstddef> includes in tests

Some tests use type std::max_align_t, but don't include <cstddef> header
directly. As a result, these tests won't compile against some conformant
libraries.

Reviewed as https://reviews.llvm.org/D54645.
Thanks to Andrey Maksimov for the patch.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347232 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed