Add pcntl_alarm to the list of impure functions

[l-x]

pcntl_alarm is currently considered as a pure function, which is not correct.

See https://psalm.dev/r/c51166d9bb

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/c51166d9bb

<?php

pcntl_alarm(123);

Psalm output (using commit 5dadb02):

ERROR: UnusedFunctionCall - 3:1 - The call to pcntl_alarm is not used

[weirdan]

Thanks!