fix(examples) don't compile assets unless needed (#2523)

This wraps the code for the assets under examples/assets in
LV_BUILD_EXAMPLES so that it is not compiled when examples are
deactivated in lv_conf.h.

examples/assets/img_hand.c

@@ -1,5 +1,7 @@
 #include "../../lvgl.h"
 
+#if LV_BUILD_EXAMPLES
+
 #ifndef LV_ATTRIBUTE_MEM_ALIGN
 #define LV_ATTRIBUTE_MEM_ALIGN
 #endif
@@ -67,3 +69,4 @@ const lv_img_dsc_t img_hand = {
   .data = img_hand_map,
 };
 
+#endif /* LV_BUILD_EXAMPLES */