openbsm: fix linux build

See https://hydra.nixos.org/build/80705916

To fix the linux build the patch `bsm-add-audit_token_to_pid.patch`
mustn't be applied during Linux builds, only for Darwin as it's an
Apple-only fix.

The compiler failure occurred because `audit_token_t` is part of
`<mach/mach.h>` which is not available on Linux.

Addresses #45960