C++: Make XMLFile extend File again

Commit a1e4404 made `XMLFile` no longer extend `File`. I'm guessing
this was necessary in the branch where `File` was an IPA-typed `Element`
and `XMLFile` was not, but it broke compilation of some of our internal
queries.

Author: jbj

cpp/ql/src/semmle/code/cpp/XML.qll

@@ -94,7 +94,7 @@ class XMLParent extends @xmlparent {
 }
 
 /** An XML file. */
-class XMLFile extends XMLParent {
+class XMLFile extends XMLParent, File {
   XMLFile() {
     xmlEncoding(this,_)
   }