openttd-jgrpp: Add optional but recommended zstd dependency

Fixes

```
CMake Warning at cmake/LinkPackage.cmake:23 (message):
  ZSTD not found; compiling OpenTTD without ZSTD is not recommended
  Call Stack (most recent call first):
    CMakeLists.txt:330 (link_package)
```

pkgs/games/openttd/jgrpp.nix

@@ -1,4 +1,4 @@
-{ fetchFromGitHub, openttd, ... }:
+{ fetchFromGitHub, openttd, zstd, ... }:
 
 openttd.overrideAttrs (oldAttrs: rec {
   pname = "openttd-jgrpp";
@@ -10,4 +10,6 @@ openttd.overrideAttrs (oldAttrs: rec {
     rev = "jgrpp-${version}";
     hash = "sha256-AMd2KXy/ODByeV9CkEd51KbE/+fZ8Us3WzsWCnn7nh0=";
   };
+
+  buildInputs = oldAttrs.buildInputs ++ [ zstd ];
 })