[PATCH] tools/widl/header.c: Add a way to declare interface data members.

[Max TenEyck Woodbury]

I mentioned this a few days ago.  It would have helped if you had
raised this point then.

As it stands, it is simply a way to adding data members to an aggregate
with an interface.  In that sense it is not an addition to the
interface since the Vtbl pointer remains exactly as before.  The new
information is not part of the interface as such.  Putting it in the
same 'struct' as the Vtbl is simply a way of enforcing the
association.  If you do not define a <iface>_IFACE_DATA macro, nothing
happens.