curl_json_test.c: fix warnings

curl_json_test.c: fix warnings

src/curl_json_test.c:142:14: warning: unused parameter 'argc' [-Wunused-parameter]
int main(int argc, char **argv) {
             ^
src/curl_json_test.c:142:27: warning: unused parameter 'argv' [-Wunused-parameter]
int main(int argc, char **argv) {
                          ^
2 warnings generated.