fix with rrdtool 1.9

Index: src/LuaEngineNtop.cpp
--- src/LuaEngineNtop.cpp.orig
+++ src/LuaEngineNtop.cpp
@@ -5427,7 +5427,7 @@ static int ntop_rrd_tune(lua_State* vm) {
   filename = argv[1];
 
   reset_rrd_state();
-  status = rrd_tune(argc, (char**)argv);
+  status = rrd_tune(argc, argv);
 
   if(status != 0) {
     char *err = rrd_get_error();
