package/qpdf: New package qpdf 5.1.2

[Thomas:
  - adjust length of comment header in .mk file.
  - fix comment in Config.in to match what's specific in the Buildroot
    manual.
  - fix <pkg>_LICENSE to 'Artistic-2.0'.]

Signed-off-by: Olivier Schonken <olivier.schonken@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>