Wrapped unity using in the C# preprocessor

Signed-off-by: Vanessa Vinther <7903603+ImDreamerDev@users.noreply.github.com>

Author: ImDreamerDev

sdks/csharp/src/Table.cs

@@ -4,7 +4,9 @@
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
+#if UNITY_5_3_OR_NEWER
 using UnityEngine;
+#endif
 using SpacetimeDB.BSATN;
 using SpacetimeDB.ClientApi;
 using SpacetimeDB.EventHandling;