lib: Fix an introspection annotation
lib: Fix an introspection annotation

The "allow-none" annotation does not make sense for a plain boolean
parameter.

Closes: #267
Approved by: cgwalters

GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree