feat(demo): add flex_layout demo (#3755)

Signed-off-by: pengyiqiang <pengyiqiang@xiaomi.com>
Signed-off-by: W-Mai <1341398182@qq.com>
Co-authored-by: Uli Raich <uli.raich@gmail.com>
Co-authored-by: W-Mai <1341398182@qq.com>
Co-authored-by: pengyiqiang <pengyiqiang@xiaomi.com>
Co-authored-by: Gabor Kiss-Vamosi <kisvegabor@gmail.com>
Co-authored-by: Man, Jianting (Meco) <920369182@qq.com>

demos/lv_demos.h

@@ -35,6 +35,10 @@ extern "C" {
 #include "widgets/lv_demo_widgets.h"
 #endif
 
+#if LV_USE_DEMO_FLEX_LAYOUT && LV_USE_USER_DATA
+#include "flex_layout/lv_demo_flex_layout.h"
+#endif
+
 /*********************
  *      DEFINES
  *********************/