Fix compilation of Samples.Security.AspNetCore5 (#6512)

## Summary of changes

Make Samples.Security.AspNetCore5 compile again.

## Implementation details

A field that was inadvertently removed was actually used.

tracer/test/test-applications/security/Samples.Security.AspNetCore5/Controllers/IastController.cs

@@ -79,6 +79,7 @@ public class IastController : Controller
     {
         private static SQLiteConnection _dbConnectionSystemData = null;
         private static SqliteConnection _dbConnectionSystemDataMicrosoftData = null;
+        private static SqlConnection _dbConnectionSystemDataSqlClient = null;
         private static IMongoDatabase _mongoDb = null;
 #if NETCOREAPP3_0_OR_GREATER
         private static NpgsqlConnection _dbConnectionNpgsql = null;