systemd: re-enable bpf-framework

systemd meson.build apparently didn't check properly that the option was
enabled in all code paths, so it was possible to build systemd such as
--version would have -BPF_FRAMEWORK (properly disabled in config.h) with
-Dbpf-framework=enabled.

Fix the failing check, which was `clang -target bpf` breaking with
zerocallusedregs hardening -- this is apparently a known problem as it's
disabled in quite a few other packages that mention bpf in comment above
the exception...

Link: https://github.com/systemd/systemd/pull/33793

pkgs/os-specific/linux/systemd/default.nix

@@ -412,7 +412,8 @@ stdenv.mkDerivation (finalAttrs: {
   hardeningDisable = [
     # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111523
     "trivialautovarinit"
-  ];
+    # breaks clang -target bpf; should be fixed to filter target?
+  ] ++ (lib.optional withLibBPF "zerocallusedregs");
 
   nativeBuildInputs =
     [