ntopng: fix missing static assets

Instead of building the `dist` make target in the derivation (requires npm and
all that), we're using the upstream pre-built static assets. Upstream refers to
these using a sub-module so we need to fetch that.

Without this patch the web UI loads the html, but 404s on all CSS and JS assets
because the `$out/share/ntopng/httpdocs/dist` directory is empty

pkgs/tools/networking/ntopng/default.nix

@@ -32,7 +32,8 @@ stdenv.mkDerivation rec {
     owner = "ntop";
     repo = "ntopng";
     rev = "refs/tags/${version}";
-    hash = "sha256-iGqrS0AneKYwGMEpbKy9if8bnaEu6aEV+QaH+JrF9xs=";
+    hash = "sha256-pIm0C1+4JLVDdXxSaQtd6ON8R2l6KG8ZXuDDuRd6dQI=";
+    fetchSubmodules = true;
   };
 
   preConfigure = ''