+<utility> for std::exchange

[al-mission-2016]

Add <utility> header for std::exchange used in this implementation file, otherwise the unit (and the whole lib consequently) cannot be compiled/build in some environments, e.g. ArchLinux/AUR -- all, as usual, depends on compilation order, compiler version etc. It compiles/builds allright (by AUR) after this patch.)

[JimmyCabbage]

Cannot build in Debian 12 without this modification, would help to have this in the main repo.