build: enable -Wimplicit-fallthrough warning for OpenCV modules

modules/core/src/persistence_base64.cpp

@@ -84,7 +84,9 @@ size_t base64_encode(uint8_t const * src, uint8_t * dst, size_t off, size_t cnt)
     switch (rst)
     {
     case 1U: *dst_cur++ = base64_padding;
+        /* fallthrough */
     case 2U: *dst_cur++ = base64_padding;
+        /* fallthrough */
     default: *dst_cur   = 0;
         break;
     }