[gn build] Port fcaccf8

Author: llvmgnsyncbot

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

@@ -537,6 +537,7 @@ if (current_toolchain == default_toolchain) {
       "__locale_dir/locale_base_api/bsd_locale_fallbacks.h",
       "__locale_dir/locale_base_api/locale_guard.h",
       "__mbstate_t.h",
+      "__mdspan/extents.h",
       "__memory/addressof.h",
       "__memory/align.h",
       "__memory/aligned_alloc.h",