widl: autogenerated headers cause warnings on MSVC

When compiling shell32 with MSVC, we get a lot of the following warnings:

output-VS-i386\reactos\include\psdk\shobjidl.h(19564) : warning C4099: 'DestinationList' : type name first seen using 'struct' now seen using 'class'

This might be benign, but it's undesireable and should be fixed in widl.