[gn build] Port 2f1416b

Author: llvmgnsyncbot

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

@@ -1455,6 +1455,8 @@ if (current_toolchain == default_toolchain) {
       "__flat_map/sorted_equivalent.h",
       "__flat_map/sorted_unique.h",
       "__flat_map/utils.h",
+      "__flat_set/flat_set.h",
+      "__flat_set/ra_iterator.h",
       "__format/buffer.h",
       "__format/concepts.h",
       "__format/container_adaptor.h",