Fix rebuilding docs, by hiding __attribute__((...)) behind a macro.

When enabled via `./configure --enable-rebuild-docs`,
`make -C doc libxml2-api.xml` will invoke apibuild.py
to rebuild libxml2-api.xml from the sources.
But the code added in
9fa3200cb3 made it error out with

```
Parsing ../parser.c
Parse Error: parsing type : expecting a name
('Got token ', ('sep', '('))
('Last token: ', ('sep', '('))
('Token queue: ', [('name', 'destructor'), ('sep', ')'), ('sep', ')')])
('Line 14689 end: ', '')
```

doc/apibuild.py

@@ -74,6 +74,7 @@ ignored_words = {
   "__declspec": (3, "Windows keyword"),
   "__stdcall": (0, "Windows keyword"),
   "ATTRIBUTE_UNUSED": (0, "macro keyword"),
+  "ATTRIBUTE_DESTRUCTOR": (0, "macro keyword"),
   "LIBEXSLT_PUBLIC": (0, "macro keyword"),
   "X_IN_Y": (5, "macro function builder"),
   "ATTRIBUTE_ALLOC_SIZE": (3, "macro for gcc checking extension"),