brook: enable on darwin

pkgs/tools/networking/brook/default.nix

···
     homepage = "https://github.com/txthinking/brook";
     description = "A cross-platform Proxy/VPN software";
     license = with licenses; [ gpl3Only ];
-    platforms = platforms.linux;
+    platforms = platforms.unix;
     maintainers = with maintainers; [ xrelkd ];
   };
 }