Fix build on DragonFlyBSD

[daftaupe]

License: MIT
Signed-off-by: Pierre-Alain TORET [email protected]

[djdv]

I'm curious if we can just omit these constraints altogether, so new platforms don't have to keep rediscovering this issue when trying to build on unsupported platforms.
To my knowledge, these files are only built when supplying the nofuse build tag anyway and the implementation for nofuse should work across all go implementations. (it seems to just return a string/error). Further limiting via GOOS seems unnecessary.

[Kubuxu]

@djdv please open issue about it, it will get lost here.