[Build][Windows] Add windows debug postfix definition

[enen92]

Allows to distinguish between release and debug builds. In Kodi we are already using exiv2 on master (future release) and we are including this patch (added in https://github.com/xbmc/xbmc/commit/501ec1fae0fb86760548018f02702a9caf8ba997) so we though about submitting upstream. cc @fuzzard