[gn build] Port 2a38551

Author: llvmgnsyncbot

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

@@ -770,6 +770,7 @@ if (current_toolchain == default_toolchain) {
       "__thread/thread.h",
       "__thread/timed_backoff_policy.h",
       "__tree",
+      "__tuple/find_index.h",
       "__tuple/make_tuple_types.h",
       "__tuple/pair_like.h",
       "__tuple/sfinae_helpers.h",