[gn build] Port 31c9c41

Author: llvmgnsyncbot

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

@@ -780,6 +780,7 @@ if (current_toolchain == default_toolchain) {
       "__thread/timed_backoff_policy.h",
       "__tree",
       "__tuple/find_index.h",
+      "__tuple/ignore.h",
       "__tuple/make_tuple_types.h",
       "__tuple/sfinae_helpers.h",
       "__tuple/tuple_element.h",