DGCharts build fails when compiling for VisionOS

* [X] I've read, understood, and done my best to follow the [*CONTRIBUTING guidelines](https://github.com/jjatie/Charts/blob/master/CONTRIBUTING.md).

## What did you do?
Porting an app using DGCharts from MacOS to VisionOS

## What did you expect to happen?
Expected DGCharts code to build and work correctly under VisionOS

## What happened instead?
Could not build the project: it seems the VisionOS platform is not currently supported

## DGCharts Environment

**DGCharts version/Branch/Commit Number:**
5.1.0
**Xcode version:**
16.2
**Swift version:**
Swift 5
**Platform(s) running DGCharts:**
MacOS 15, iOS 18, visionOS 2
**macOS version running Xcode:**
MacOS 15.3

## Demo Project
No need to add demo project, since the library clearly does not support the visionOS platform