add libpng dependency explicitly

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-webservice.

I've rerendered the recipe as instructed in #10.

Here's a checklist to do before merging.

• [ ] Bump the build number if needed.

Fixes #10

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[ebolyen]

Thank you @isuruf!

[bgruening]

Thanks all!