mstask: Added IEnumWorkItems stub

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=12318

Your paranoid android.


=== w2000pro (32 bit task_scheduler) ===
task_scheduler: unhandled exception c0000005 at 6AC2AEEF