[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

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 };.

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

@@ -13,6 +13,7 @@
 
 #include <array>
 #include <cassert>
+#include <cstddef>       // for std::max_align_t
 
 #include "test_macros.h"
 

test/std/utilities/memory/default.allocator/allocator.members/allocate.pass.cpp

@@ -14,6 +14,7 @@
 
 #include <memory>
 #include <cassert>
+#include <cstddef>       // for std::max_align_t
 #include <iostream>
 
 #include "test_macros.h"