Add missing function definition for lv_group_del

lv_core/lv_group.h

@@ -62,6 +62,12 @@ typedef struct _lv_group_t
 lv_group_t * lv_group_create(void);
 
 /**
+ * Delete a group object
+ * @param group pointer to a group
+ */
+void lv_group_del(lv_group_t * group)
+
+/**
  * Add an object to a group
  * @param group pointer to a group
  * @param obj pointer to an object to add