Fix build with boost-1.85

[stefantalpalaru]

No description provided.

[Tabby]

This header has "only" been deprecated since Boost 1.50 (though I'd guess it got deprecated as part of the move to Filesystem V3 which happened earlier than that, it's just that V2 was kept around for a while for compatibility).

Even though that version is about 12 years old, ElectricSheep still lists the even older 1.39 as the minimum version, so this change should probably either bump the minimum version, or conditionally change which includes are used based on the version of Boost in use

[Schievel1]

I think we shouldn't bump the minimum version, as this would break compilation for many older systems. After all boost 1.85 is only a few months old.
I made a PR to include the right file depending on the boost version.