posix: signal: type corrections for sigval, sigevent, notify attr

* `struct sigevent` is not type-defined
* `union sigval` is not type-defined
* `struct sigevent` must include `sigev_notify_attributes`

For more information, see https://bit.ly/3YfnELI



Signed-off-by: Christopher Friedt <cfriedt@meta.com>