print/pdftk: Update to 3.0.8 of JDK-based fork

The fork is a pure Java implementation of pdftk and does not rely
on the obsolete GCJ. It is mostly compatible with the old version.
The old version was the last port still depending on lang/gcc6.

Changes: https://gitlab.com/pdftk-java/pdftk/blob/v3.0.8/CHANGELOG.md#308-2019-10-14