Poppler on Android builds

[Aleix Pol]

 Hey,
I guess you knew there are Okular Android builds:
https://binary-factory.kde.org/view/Android/job/Okular_android/

Build is broken now because the Android NDK doesn't have std::stoi.
https://binary-factory.kde.org/view/Android/job/Okular_android/146/console

I'll change it to build the poppler-0.68.0 tag for now.

Hope that makes sense to you, tell me otherwise.
Aleix