Fix CurrentTimestampSelectString for Firebird4Dialect

Author: hazzik

src/NHibernate/Dialect/Firebird4Dialect.cs

@@ -4,6 +4,8 @@ namespace NHibernate.Dialect
 {
 	public class Firebird4Dialect: FirebirdDialect
 	{
+		public override string CurrentTimestampSelectString => "select LOCALTIMESTAMP from RDB$DATABASE";
+
 		protected override void RegisterFunctions()
 		{
 			base.RegisterFunctions();