vk.xml's `define` category no longer restricted to actual #defines

Vulkan 1.1 added the following to vk.xml:

struct ANativeWindow;

This is not a `#define`, yet it uses the `define` category. It ought to be a `basetype` (not that that would make processing it easier). I imagine `define` was used because it worked, because the C and C++ generators just dump whatever is in a `define` category without processing it.

If the idea is that `define` is just "random C/C++ stuff", then that should probably be well-stated in the documentation. But at present, all it says is "`define` (macro `#define` directives)".