tool_operate: fix compiler warning when --libcurl is disabled

Closes #6095

src/tool_operate.c

@@ -322,6 +322,9 @@ static CURLcode pre_transfer(struct GlobalConfig *global,
 
     if(uploadfilesize != -1) {
       struct OperationConfig *config = per->config; /* for the macro below */
+#ifdef CURL_DISABLE_LIBCURL_OPTION
+      (void)config;
+#endif
       my_setopt(per->curl, CURLOPT_INFILESIZE_LARGE, uploadfilesize);
     }
     per->input.fd = per->infd;