graphics/gscan2pdf: update to 2.13.5

Now also supports ImageMagick7 (follows DEFAULT_VERSIONS)

Announcement:

https://sourceforge.net/p/gscan2pdf/mailman/message/59255852/

Never mind the typo in the announcement, its actually 2.13.5:

https://sourceforge.net/p/gscan2pdf/code/ci/v2.13.5/tree/