C++: add .def to exceptions to AV rule 32

This is used as textual includes in several projects for macro
metaprogramming, for example in `llvm-project` and in `swift` (and since
some time in our internal codebase as well).

cpp/ql/src/jsf/4.06 Pre-Processing Directives/AV Rule 32.ql

@@ -18,6 +18,7 @@ from Include i, File f, string extension
 where
   f = i.getIncludedFile() and
   extension = f.getExtension().toLowerCase() and
+  extension != "def" and
   extension != "inc" and
   extension != "inl" and
   extension != "tcc" and