[gn build] Port d6832a6

Author: llvmgnsyncbot

llvm/utils/gn/secondary/libcxx/include/BUILD.gn

@@ -396,6 +396,11 @@ if (current_toolchain == default_toolchain) {
       "__coroutine/coroutine_traits.h",
       "__coroutine/noop_coroutine_handle.h",
       "__coroutine/trivial_awaitables.h",
+      "__cstddef/byte.h",
+      "__cstddef/max_align_t.h",
+      "__cstddef/nullptr_t.h",
+      "__cstddef/ptrdiff_t.h",
+      "__cstddef/size_t.h",
       "__debug_utils/randomize_range.h",
       "__debug_utils/sanitizers.h",
       "__debug_utils/strict_weak_ordering_check.h",