[Toybox] [PATCH] Fix xsignal_all_killers().

[enh]

Found in github CI by the existing patch tests:
```
patch: xsignal 0: Invalid argument
```

When I grepped for places that would be affected by my `kill -0` change,
I grepped toys/, not lib/. (And I only ran the tests for the toys I
touched, not all the tests, and CI was already broken so we didn't
notice the new breakage until the old breakage was fixed :-( )
---
 lib/portability.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
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