I wouldnt advise you to try and get away with it mate,
Suppose it depends how much of the post is in the frame to support it, i know 0.2mm aint a lot but it might be enough with prolonged use to do some damage ie crack the collar area due to over tightening to take up the "slack".
Better to try and get a trade for the same post in the correct size to be honest.
Well. 0.2mm is just the right difference to be taken up with a Coke can shim .
Make it as long as possible (80-100mm) and the right circumference (a gnat's cock uner 85mm). De-grease the inside of the seatube and bond the shim in position ( with the seatpost in place while you do this) with Araldite. Leave 2mm of the shim proud of the seat tube. When it has cured, remove the post and form the overhanging section into a "lip" over the top of the seat tube.
If you never alter the position of the seatpost in the frame you could forego the bonding etc. but you want to drop the seatpost then bond it, otherwise the shim probably won't say in place.
If you ever want to remove it you will be able to peel it out without much trouble.