ghostscript: security bump to version 9.25

ghostscript: security bump to version 9.25

Fixes the following security issues:

- CVE-2018-16543: In Artifex Ghostscript before 9.24, gssetresolution and
  gsgetresolution allow attackers to have an unspecified impact

- CVE-2018-17183: Artifex Ghostscript before 9.25 allowed a user-writable
  error exception table, which could be used by remote attackers able to
  supply crafted PostScript to potentially overwrite or replace error
  handlers to inject code.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>