Fix build with Qt <5.10

Summary:
Qt::AA_CompressTabletEvents was added in Qt 5.10. Use preprocessor check
to avoid build failures on 5.9 LTS or 5.8 (the minimum version).

Test Plan: Build against Qt 5.9 LTS and 5.12 LTS. Both built successfully.

Subscribers: okular-devel

Tags: Okular

Differential Revision: https://phabricator.kde.org/D20678