ghostscript: add patch for CVE-2019-6116

This is tagged as version 9.26a in the ghostpdl repo, but unfortunately
there are no tarballs released with that version number so far. We'll
continue calling this version 9.26 for now for simplicity's sake (and we
can switch to 9.26a and remove the patch when it's properly released).

Fixes #58262
Fixes #58089