Suppress deprecated warnings for g_thread_create()

    milter-test-server.c: In function ‘main’:
    milter-test-server.c:2342:13: warning: ‘g_thread_create’ is deprecated (declared at /usr/include/glib-2.0/glib/deprecated/gthread.h:100): Use 'g_thread_new' instead [-Wdeprecated-declarations]
                 threads[i] = g_thread_create(test_server_thread, &process_data[i], TRUE, NULL);
                 ^

GitHub: see #32