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

pkgs/development/libraries/openbsm/default.nix

···
     sha256 = "0b98359hd8mm585sh145ss828pg2y8vgz38lqrb7nypapiyqdnd1";
   };
 
-  patches = [ ./bsm-add-audit_token_to_pid.patch ];
+  patches = lib.optional stdenv.isDarwin [ ./bsm-add-audit_token_to_pid.patch ];
 
   meta = {
     homepage = http://www.openbsm.org/;