pax-utils: enable seccomp and fuzzong only on linux

On `darwin` the build fails on missing `memfd_create()` symbols as
https://hydra.nixos.org/build/314413362:

    ../fuzz-ar.c:18:7: error: call to undeclared function 'memfd_create'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
       18 |         fd = memfd_create("fuzz-input.a", MFD_CLOEXEC);
          |              ^

pkgs/by-name/pa/pax-utils/package.nix

@@ -5,7 +5,9 @@
   buildPackages,
   docbook_xml_dtd_44,
   docbook_xsl,
+  withFuzzing ? stdenv.hostPlatform.isLinux,
   withLibcap ? stdenv.hostPlatform.isLinux,
+  withSeccomp ? stdenv.hostPlatform.isLinux,
   libcap,
   pkg-config,
   meson,
@@ -29,7 +31,9 @@ stdenv.mkDerivation rec {
   strictDeps = true;
 
   mesonFlags = [
+    (lib.mesonBool "use_fuzzing" withFuzzing)
     (lib.mesonEnable "use_libcap" withLibcap)
+    (lib.mesonBool "use_seccomp" withSeccomp)
   ];
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];