libbpf: fix build with structuredAttrs

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

@@ -39,7 +39,7 @@ stdenv.mkDerivation rec {
   enableParallelBuilding = true;
   makeFlags = [
     "PREFIX=$(out)"
-    "-C src"
+    "--directory=src"
   ];
 
   passthru.tests = {
@@ -59,6 +59,8 @@ stdenv.mkDerivation rec {
 
   # outputs = [ "out" "dev" ];
 
+  __structuredAttrs = true;
+
   meta = {
     description = "Library for loading eBPF programs and reading and manipulating eBPF objects from user-space";
     homepage = "https://github.com/libbpf/libbpf";