meson: Fix no-X11 documentation build
meson: Fix no-X11 documentation build

https://bugzilla.gnome.org/show_bug.cgi?id=794872

Signed-off-by: Quentin Glidic <241385a0e41ea713327e189bce1062decf7c1eb5@sardemff7.net>

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf