fix: do not pass graphviz as a dependency

On mipsel, we do not have… graphviz.

Signed-off-by: Raito Bezarius <masterancpp@gmail.com>

overlay.nix

@@ -191,6 +191,10 @@ extraPkgs // {
     });
   in h.override { openssl = null; sqlite = null; };
 
+  libnl = prev.libnl.override {
+    graphviz = null;
+  };
+
   wpa_supplicant = prev.wpa_supplicant.override {
     dbusSupport = false;
     withPcsclite = false;