handle htobe32 et al. on glibc systems with a non-Linux kernel

This fix copies over an updated #if clause from libknot's src/contrib/endian.h.

This should resolve:

https://gitlab.labs.nic.cz/knot/knot-resolver/issues/348

See also discussion about this same problem in libknot from a couple years ago:

https://bugs.debian.org/840460

I note that contrib/wire.h in knot-resolver is out of sync with src/contrib/wire.h (and src/contrib/endian.h) from libknot. I don't know whether there's any upstream preference for keeping these in sync in some more reliable way than manual comparisons. For now i'm just providing a narrow fix for the specific problem.