Support raw pointers in cxx::bridge.

This allows raw pointers to types to be passed into and out of
cxx::bridge extern "C++" APIs. As normal with raw pointers in Rust,
there are no safety or lifetime guarantees.

Passing a raw pointer into such an API requires that the
function be marked "unsafe".
diff --git a/syntax/pod.rs b/syntax/pod.rs
index e1c4831..0bf152e 100644
--- a/syntax/pod.rs
+++ b/syntax/pod.rs
@@ -29,7 +29,7 @@
             | Type::WeakPtr(_)
             | Type::CxxVector(_)
             | Type::Void(_) => false,
-            Type::Ref(_) | Type::Str(_) | Type::Fn(_) | Type::SliceRef(_) => true,
+            Type::Ref(_) | Type::Str(_) | Type::Fn(_) | Type::SliceRef(_) | Type::Ptr(_) => true,
             Type::Array(array) => self.is_guaranteed_pod(&array.inner),
         }
     }