serd: enable on darwin

pkgs/development/libraries/serd/default.nix

@@ -16,6 +16,6 @@ stdenv.mkDerivation rec {
     description = "A lightweight C library for RDF syntax which supports reading and writing Turtle and NTriples";
     license = licenses.mit;
     maintainers = [ maintainers.goibhniu ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
   };
 }