Additions to Makefile for cleaning up OpenRA.Game/*.dll left-overs

followup of https://github.com/OpenRA/OpenRA/pull/5089 after `git status` detected these:

```
        OpenRA.Game/ICSharpCode.SharpZipLib.dll
        OpenRA.Game/OpenRA.FileFormats.dll
        OpenRA.Game/OpenRA.FileFormats.dll.mdb
        OpenRA.Game/SharpFont.dll
        OpenRA.Game/Tao.OpenAl.dll
```