drmodtrack offset field has uninitialized values on Windows

For #2939 (closed) an offset field was added to drmodtrack in PR #2940 (and PR #2973). However, it is ifdef-ed out for Windows in some places but not in others. Now, I'm adding a new field for #4474, and drmodtrack-test is failing with uninit bogus printed offset values differing bet online and offline. Not sure how it passed before: probably just got lucky and the uninit values matched.

It's too late to ifdef it out of the interface so maybe the best solution is to set it to 0 on Windows.