Revert "Zenity has deprecated --icon-name in favor of --icon"

This reverts commit 55c3c1b05c387d87b64b981ca9883fa36d7ee951.

It turns out many Linux distributions are shipping zenity which doesn't support the new --icon option. We'll need a more robust fix for this.

src/video/wayland/SDL_waylandmessagebox.c

@@ -60,7 +60,7 @@ int Wayland_ShowMessageBox(const SDL_MessageBoxData *messageboxdata, int *button
             _exit(128);
         }
 
-        argv[argc++] = "--icon";
+        argv[argc++] = "--icon-name";
         switch (messageboxdata->flags) {
         case SDL_MESSAGEBOX_ERROR:
             argv[argc++] = "dialog-error";