Make vtables non-zero-size to fix a rustc warning.

```
warning: found non-foreign-function-safe member in struct marked #[repr(C)]: found zero-size struct in foreign module, consider adding a member to this struct
```

1 files changed, 3 insertions, 1 deletions

diff --git a/src/codegen/mod.rs b/src/codegen/mod.rs
index 577b8aab..fb6c839d 100644
--- a/src/codegen/mod.rs
+++ b/src/codegen/mod.rs
@@ -712,7 +712,9 @@ impl<'a> CodeGenerator for Vtable<'a> {
             .item()
             .pub_()
             .with_attrs(attributes)
-            .struct_(self.canonical_name(ctx))
+            .tuple_struct(self.canonical_name(ctx))
+            .field()
+            .build_ty(helpers::ast_ty::raw_type(ctx, "c_void"))
             .build();
         result.push(vtable);
     }