[libc] Correct hidden attribute on __restore_rt (#220039)

This PR fixes the syntax for a `hidden` attribute in `__restore_rt`.
When compiling with `-Wattributes`, this TU complains about:

```
llvm-project/libc/src/signal/linux/__restore.cpp:20:67: error: unknown attribute 'hidden' ignored [-Werror,-Wunknown-attributes]
   20 | extern "C" void __restore_rt() __attribute__((no_sanitize("all"), hidden));
```

GitOrigin-RevId: 755212d8134e1ac4f583bd8ff40870bfb9f73978