[poppler] fix build with gtk-doc 1.27

[David Tardon]

Hello,

building with ENABLE_GTK_DOC fails with gtk-doc 1.27 (I'm pretty sure the
culprit is https://git.gnome.org/browse/gtk-doc/commit/?id=775e4a467dc448a130d33ddcc35a879f8dfb5e97).
The attached patch fixes that.

D.
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