kcachegrind: fix graphviz missing at runtime

pkgs/kde/gear/kcachegrind/default.nix

···
 {
+  lib,
   mkKdeDerivation,
   qttools,
+  graphviz,
 }:
 mkKdeDerivation {
   pname = "kcachegrind";
 
   extraNativeBuildInputs = [qttools];
+
+  qtWrapperArgs = [
+    "--suffix PATH : ${lib.makeBinPath [ graphviz ]}"
+  ];
 }