kitinerary: fix compilation with poppler >= 0.83

Summary:
state->getPath() now returns a const GfxPath, and the methods
we are calling on it are indeed const now.

But they were not const before, so we can't just use const
unconditionally here, hence the version check and the #define.

Test Plan: Builds

Reviewers: vkrause, aacid

Reviewed By: vkrause

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D25644