Make 'gb.pdf' compile with poppler-0.72
completes commit a576b687
Without this change, it would complain about missing getCString because the POPPLER_VERSION_0_72 was not there.
Not sure why this was not failing before - maybe related to openSUSE's recent glibc-2.29 upgrade? Or we only upgraded poppler to 0.72 in December and maybe I did not try since then.