fix(draw_rect): delete __STDC_VERSION__ to ensure C++ compatibility (#3099)

src/draw/lv_draw_rect.c

@@ -38,9 +38,7 @@ LV_ATTRIBUTE_FAST_MEM void lv_draw_rect_dsc_init(lv_draw_rect_dsc_t * dsc)
 {
     lv_memset_00(dsc, sizeof(lv_draw_rect_dsc_t));
     dsc->bg_color = lv_color_white();
-#if __STDC_VERSION__ < 201112L
     dsc->bg_grad.stops[0].color = lv_color_white();
-#endif
     dsc->bg_grad.stops[1].color = lv_color_black();
     dsc->bg_grad.stops[1].frac = 0xFF;
     dsc->bg_grad.stops_count = 2;

src/draw/lv_draw_rect.h

@@ -35,14 +35,8 @@ typedef struct {
 
     /*Background*/
     lv_opa_t bg_opa;
-#if __STDC_VERSION__ >= 201112L
-    union {
-#endif
-        lv_color_t bg_color;        /**< First element of a gradient is a color, so it maps well here*/
-        lv_grad_dsc_t bg_grad;
-#if __STDC_VERSION__ >= 201112L
-    };
-#endif
+    lv_color_t bg_color;        /**< First element of a gradient is a color, so it maps well here*/
+    lv_grad_dsc_t bg_grad;
 
     /*Background img*/
     const void * bg_img_src;