Add an explicit coercion so that it compiles with both lablgtk 2.16 and 2.18.
Add an explicit coercion so that it compiles with both lablgtk 2.16 and 2.18.

ssaavedra/why3,jpdeplaix/why3,ssaavedra/why3,douglas-larocca/why3,jpdeplaix/why3,ssaavedra/why3,douglas-larocca/why3,douglas-larocca/why3,jpdeplaix/why3