Hi
You should read those licences to find out 
We don’t ship the licence for libpng because we don’t ship libpng, only our Windows build includes a binary libpng.dll and the libpng licence makes no mention of having to include the licence if one doesn’t ship the source.
Our licences are here:
If a licence is missing, feel free to open an issue explicitly stating why you think that licence is missing and referencing a specific section of the relevant licence or otherwise binding document which instructs users to include it when shipping the source code or when shipping a binary-only form.