folly/executors/QueueObserver.h:54:17: error: use of undeclared identifier 'pid_t'

```
In file included from /wrkdirs/usr/ports/devel/folly/work/folly-2021.08.23.00/folly/executors/CPUThreadPoolExecutor.cpp:18:
In file included from /wrkdirs/usr/ports/devel/folly/work/folly-2021.08.23.00/folly/executors/CPUThreadPoolExecutor.h:23:
/wrkdirs/usr/ports/devel/folly/work/folly-2021.08.23.00/folly/executors/QueueObserver.h:54:17: error: use of undeclared identifier 'pid_t'
std::vector threadIds;
^
```

@ https://github.com/facebook/folly/blob/master/folly/executors/QueueObserver.h#L54

Did you forget to include ```signal.h```?

folly-2021.08.23.00
OS: FreeBSD 13

Start at folly/executors/QueueObserver.h:54 and inspect how pid_t is declared on FreeBSD 13, including the header dependencies used by this file. Reproduce the folly-2021.08.23.00 build failure on FreeBSD, then verify that the correction allows CPUThreadPoolExecutor.cpp to compile without breaking other platforms.

Written by the indexing model from the issue text.