Compile tablet driver benchmark only with .NET 5

osu.Framework.Benchmarks/BenchmarkTabletDriver.cs

···
 // Copyright (c) ppy Pty Ltd <[e-mail]>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
+#if NET5_0
 using BenchmarkDotNet.Attributes;
 using osu.Framework.Input.Handlers.Tablet;
 
···
         }
     }
 }
+#endif