ei: declare struct ei_touch

Part-of: <https://gitlab.freedesktop.org/libinput/libei/-/merge_requests/313>

src/libei.h

@@ -205,6 +205,18 @@ struct ei_keymap;
  */
 struct ei_region;
 
+/**
+ * @struct ei_touch
+ *
+ * A single touch initiated by a sender context.
+ *
+ * @see ei_device_touch_new
+ * @see ei_touch_down
+ * @see ei_touch_motion
+ * @see ei_touch_up
+ */
+struct ei_touch;
+
 /**
  * @enum ei_device_type
  * @ingroup libei-device