[gn build] Port f22ecdd

Author: llvmgnsyncbot

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

@@ -432,6 +432,7 @@ if (current_toolchain == default_toolchain) {
       "__filesystem/space_info.h",
       "__filesystem/u8path.h",
       "__flat_map/flat_map.h",
+      "__flat_map/key_value_iterator.h",
       "__flat_map/sorted_unique.h",
       "__format/buffer.h",
       "__format/concepts.h",