Minor fix to compile on FreeBSD

[stackyjoe]

See issue #202

[Geod24]

Is this ready ?

[herrhotzenplotz]

Just chiming in here, while being confronted with exactly this issue:

the patch perfectly solves the compilation failure on the following (and other test systems):

$ dmd --version
DMD64 D Compiler v2.098.0-40-g54fa6ee34
Copyright (C) 1999-2021 by The D Language Foundation, All Rights Reserved written by Walter Bright
$ uname -omr
FreeBSD 13.0-RELEASE-p6 amd64
$ 

Changes are not causing any trouble at runtime and things are running stable for me.
I'd be glad if this gets merged as quickly as possible.

[s-ludwig]

The only question I'd have is whether this is changing anything on macOS, because that is also changed to use lseek - keeping the old call in another version (OSX) block would avoid that potential issue.