ol32: Make enumx implementation look more like a COM interface.

[Dmitry Timoshkov]

This patch should be quite obvious in its intent: it changes vtable type
to IUnknown which helps to remove This* to IUnknown* casts in order to
avoid obvious problems with vtable pointer placement inside of object
implementation. Basically this is just a clean up patch.

Is there anything else to clear the pending patch state?

-- 
Dmitry.