This is actually a rule that bugs me and I have not seen a specific resolution to the question.
Jink can be used against any 'shooting attack'.
An 'overwatch attack' is resolved like a 'shooting attack' with some exceptions.
They do not list Jink as an exception.
That said - they also call it an 'overwatch attack' and not a 'shooting attack'.
The general opinion appears to be that, yes, you may jink against overwatch.
I would always check with the judge at any tournament to be certain though - as I do see it as a grey area.