[gn build] Port 5098b56

Author: llvmgnsyncbot

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

@@ -952,6 +952,7 @@ if (current_toolchain == default_toolchain) {
       "__utility/priority_tag.h",
       "__utility/private_constructor_tag.h",
       "__utility/rel_ops.h",
+      "__utility/scope_guard.h",
       "__utility/small_buffer.h",
       "__utility/swap.h",
       "__utility/to_underlying.h",