IWYU: mark <cstddef> as needed

Newer IWYU is not seeing this as necessary for `_GLIBCXX_USE_CXX11_ABI`.

Tests/CMakeLib/testString.cxx

@@ -1,7 +1,7 @@
 /* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
    file Copyright.txt or https://cmake.org/licensing for details.  */
 
-#include <cstddef>
+#include <cstddef> // IWYU pragma: keep
 #include <cstring>
 #include <iostream>
 #include <iterator>