linux-pam: disable audit when cross-compiling from darwin (#438635)

pkgs/by-name/li/linux-pam/package.nix

···
   gettext,
   ninja,
   audit,
+  linuxHeaders,
   libxcrypt,
   nixosTests,
   meson,
···
   docbook_xsl_ns,
   nix-update-script,
   withLogind ? lib.meta.availableOn stdenv.hostPlatform systemdLibs,
-  withAudit ? lib.meta.availableOn stdenv.hostPlatform audit,
+  withAudit ?
+    lib.meta.availableOn stdenv.hostPlatform audit
+    # cross-compilation only works from platforms with linux headers
+    && lib.meta.availableOn stdenv.buildPlatform linuxHeaders,
 }:
 
 stdenv.mkDerivation (finalAttrs: {