v8: add missing ',' in OpenBSD's 'sources' section.

Add stack_trace_posix to 'sources' as well.

Fixes https://github.com/nodejs/node/issues/15784
Fixes https://github.com/nodejs/help/issues/992

add stack_trace_posix to OpenBSD's sources

PR-URL: https://github.com/nodejs/node/pull/18448


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>