bgpdump: fix cross compilation

pkgs/tools/networking/bgpdump/default.nix

···
     sha256 = "sha256-1HXMf9mHManR7jhonU2Agon0YFXOlM9APIN1Zm840AM=";
   };
 
+  postPatch = ''
+    substituteInPlace Makefile.in --replace 'ar r libbgpdump.a' '$(AR) r libbgpdump.a'
+  '';
+
   nativeBuildInputs = [ autoreconfHook ];
   buildInputs = [ zlib bzip2 ];