[gn build] Port a595b93

Author: llvmgnsyncbot

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

@@ -686,6 +686,7 @@ if (current_toolchain == default_toolchain) {
       "__ranges/views.h",
       "__ranges/zip_view.h",
       "__split_buffer",
+      "__std_mbstate_t.h",
       "__string/char_traits.h",
       "__string/constexpr_c_functions.h",
       "__string/extern_template_lists.h",