[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.

llvm-svn: 347232

libcxx/test/std/containers/sequences/array/array.data/data.pass.cpp

@@ -13,6 +13,8 @@
 
 #include <array>
 #include <cassert>
+#include <cstddef>       // for std::max_align_t
+
 #include "test_macros.h"
 
 // std::array is explicitly allowed to be initialized with A a = { init-list };.