[TravisCI] use -net-lib cs_drivers/System.Xml.dll@std for C# unsafe test

tests/unit/compile-cs-unsafe-travis.hxml

@@ -1,7 +1,7 @@
 compile-cs-unsafe.hxml
 
 -net-lib cs_drivers/System.Data.dll
--net-lib cs_drivers/System.Xml.dll
+-net-lib cs_drivers/System.Xml.dll@std
 -net-lib cs_drivers/Mono.Data.Sqlite.dll
 
 -D travis