Re-introduce -DHAVE_BOOST to cppcheck builds

[matt335672]

A regression introduced in cppcheck 2.15.0 has been fixed in 2.16.0 - see @firewave's comment in #3291

[firewave]

This did not rebuild the binary since your forgot to adjust the cache key.

[matt335672]

That's a good point - thanks.

The binary did get rebuilt for my branch, as I used a different branch name:-

https://github.com/matt335672/xrdp/actions/runs/11858253988/job/33048444115

So I'm pretty sure the modification above is fine.

I've just deleted the old cache at https://github.com/neutrinolabs/xrdp/actions/caches (see https://stackoverflow.com/questions/63521430), so I think we're all OK now, at least as far as this repository is concerned. Users with forks may also need to do this for branches they are working on.