diff --git a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
index e865c61968748..53d78e0365a8f 100644
--- a/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
+++ b/llvm/utils/gn/secondary/libcxx/include/BUILD.gn
@@ -611,6 +611,7 @@ if (current_toolchain == default_toolchain) {
       "__memory_resource/unsynchronized_pool_resource.h",
       "__mutex/lock_guard.h",
       "__mutex/mutex.h",
+      "__mutex/once_flag.h",
       "__mutex/tag_types.h",
       "__mutex/unique_lock.h",
       "__node_handle",