backend/x11: fix initial value of wlr_x11_buffer.n_busy

We lock the buffer there, so we need to initialize the n_busy
count to 1 as well.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3556
(cherry picked from commit bc8260f377)

backend/x11/output.c

@@ -250,6 +250,7 @@ static struct wlr_x11_buffer *create_x11_buffer(struct wlr_x11_output *output,
 		return NULL;
 	}
 	buffer->buffer = wlr_buffer_lock(wlr_buffer);
+	buffer->n_busy = 1;
 	buffer->pixmap = pixmap;
 	buffer->x11 = x11;
 	wl_list_insert(&output->buffers, &buffer->link);