Fix xmlSymbols in Null wart for scalac 2.11

Before, when a project using scala 2.11 did not use:

```scala
"org.scala-lang.modules" %% "scala-xml" % whatever
```

an exception would be thrown at the beginning of its compilation:

```
[error] (root/compile:compile) scala.ScalaReflectionException: object scala.xml.Elem in compiler mirror not found.
```
Merge pull request #90 from michalrus/fix-xml-2.11

Fix xmlSymbols in Null wart for scalac 2.11