Change stack_depth_t to size_t on OpenBSD

[dspx-plcr]

I'm not sure when OpenBSD made this change, but on OpenBSD 7.6, backtrace returns a size_t. Prior to this change, building pony fails with

/home/despereaux/build/ponyc/src/libponyrt/platform/ponyassert.c:45:25: error: implicit conversion loses i
nteger precision: 'size_t' (aka 'unsigned long') to 'stack_depth_t' (aka 'int') [-Werror,-Wshorten-64-to-3
2]
  stack_depth_t depth = backtrace(addrs, 256);
                ~~~~~   ^~~~~~~~~~~~~~~~~~~~~
1 error generated.
*** Error 1 in target 'src/libponyrt/CMakeFiles/libponyrt.dir/platform/ponyassert.c.o'
*** Error 1 in . (src/libponyrt/CMakeFiles/libponyrt.dir/build.make:612 'src/libponyrt/CMakeFiles/libponyr
t.dir/platform/ponyassert.c.o': cd...)
*** Error 2 in . (CMakeFiles/Makefile2:371 'src/libponyrt/CMakeFiles/libponyrt.dir/all')
*** Error 2 in /home/despereaux/build/ponyc/build/build_release (Makefile:136 'all': /usr/bin/make -s -f C
MakeFiles/Makefile2 all)
gmake: *** [Makefile:192: build] Error 2

[ponylang-main]

Hi @dspx-plcr,

The changelog - changed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 4575.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@dspx-plcr for the release notes on this, can you note that it was a change to support compiling on OpenBSD 7.6 and that earlier versions of OpenBSD might no longer compile (or won't compile depending on what is true).

Thanks.

[dspx-plcr]

I took a look at another PR for the release notes, let me know if you were after something a little different.

The backtrace return should silently get promoted from an int to a size_t, so I think compiling will still work, but I have not tested this myself

[SeanTAllen]

Approving with the assumption that CI will pass.

[SeanTAllen]

Thanks @dspx-plcr. A most excellent first contribution.