Add lv_deinit function (#1319)

src/lv_core/lv_obj.h

@@ -269,6 +269,15 @@ typedef struct
  */
 void lv_init(void);
 
+
+/**
+ * Deinit the 'lv' library
+ * Currently only implemented when not using custorm allocators, or GC is enabled.
+ */
+#if LV_ENABLE_GC || !LV_MEM_CUSTOM
+void lv_deinit(void);
+#endif
+
 /*--------------------
  * Create and delete
  *-------------------*/