D21320: Fix build with poppler < 0.51

[Alexander Volkov]

volkov added a reviewer: Okular.

REPOSITORY
  R223 Okular

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

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