Require Boost.Iostreams

requested review from @jbrost

Rationale: https://github.com/Icinga/icinga2/pull/9451

What's your plan here on when to merge? I think it would be cleaner to merge it together with the PR in icinga2, on the other hand, the change here on its own does not add the new dependency to the binary package, so merging it now also wouldn't hurt.

approved this merge request

I'd like to get GHA running first.

merged

mentioned in commit a5e431be

That would also have been possibly by using a different branch there temporarily.

Nice hack.