When building makefs on non-FreeBSD we get various compiler errors
including missing include files. Having makefs buildable on non-FreeBSD
is useful for building disk images - for example, the CheriBSD CI system
bootstraps makefs as part of buildworld on a Linux host.
Ideally, we would also make zfs disk image support available on non-FreeBSD
but for now this minimal change allows continued use of makefs to create
disk image on non-FreeBSD.