graphics/nxmu: Correct a compile error introduced with last big set of changes.

graphics/nxmu/nx_start.c

@@ -190,8 +190,9 @@ int nx_start(void)
                               CONFIG_NXSTART_SERVERSTACK, nx_server, NULL);
       if (server < 0)
         {
-          gerr("ERROR: Failed to create nx_server kernel thread: %d\n", ret);
-          return ret;
+          gerr("ERROR: Failed to create nx_server kernel thread: %d\n",
+               (int)server);
+          return (int)server;
         }
 
       g_nxserver_started = true;