Return UnsupportedItem when `volatile` qualifier is used.

This CL is only loosely related to b/217419782 - I've just noticed that
`volatile` is ignored when working on Importer::ConvertType changes to
support function pointer types.

PiperOrigin-RevId: 428770476
diff --git a/rs_bindings_from_cc/importer.cc b/rs_bindings_from_cc/importer.cc
index d395d98..7ba5a84 100644
--- a/rs_bindings_from_cc/importer.cc
+++ b/rs_bindings_from_cc/importer.cc
@@ -898,10 +898,12 @@
     return error;
   }
 
-  // Add cv-qualification.
+  // Handle cv-qualification.
   type->cc_type.is_const = qual_type.isConstQualified();
-  // Not doing volatile for now -- note that volatile pointers do not exist in
-  // Rust, though volatile reads/writes still do.
+  if (qual_type.isVolatileQualified()) {
+    return absl::UnimplementedError(
+        absl::StrCat("Unsupported `volatile` qualifier: ", type_string));
+  }
 
   return *std::move(type);
 }