D21320: Fix build with poppler < 0.51

[Oliver Sander]

sander accepted this revision.
This revision is now accepted and ready to land.

REPOSITORY
  R223 Okular

BRANCH
  Applications/19.04

REVISION DETAIL
  https://phabricator.kde.org/D21320

To: volkov, #okular, sander
Cc: okular-devel, joaonetto, tfella, ngraham, darcyshen, aacid
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