mark arg of compmandelbrot as unused to suppress compiler warning

multibrot.c

@@ -93,7 +93,7 @@ pthread_mutex_t next_y_mutex;
  *
  * @param arg (not used, but necessary for pthread_create.)
  */
-void *compmandelbrot(void *arg) {
+void *compmandelbrot(void *arg __attribute__((__unused__))) {
   size_t y = 0;
   while (y < mbrot->height) {
     int ret;