[gn build] Port c172082

Author: llvmgnsyncbot

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

@@ -290,6 +290,7 @@ if (current_toolchain == default_toolchain) {
       "__atomic/check_memory_order.h",
       "__atomic/contention_t.h",
       "__atomic/fence.h",
+      "__atomic/floating_point_helper.h",
       "__atomic/is_always_lock_free.h",
       "__atomic/kill_dependency.h",
       "__atomic/memory_order.h",