fix(rt-thread): fix compiler cannot find the lvgl/lvgl.h file with SquareLine (#3814)

env_support/rt-thread/squareline/lv_ui_entry.c

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2006-2022, RT-Thread Development Team
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ * Change Logs:
+ * Date           Author        Notes
+ * 2022-05-13     Meco Man      First version
+ */
+
+#ifdef __RTTHREAD__
+
+void lv_user_gui_init(void)
+{
+    extern void ui_init(void);
+    ui_init();
+}
+
+#endif /* __RTTHREAD__ */