trivial: Fix a tiny introspection warning
trivial: Fix a tiny introspection warning

ximion/appstream-glib,ximion/appstream-glib,ximion/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,hughsie/appstream-glib,ikeydoherty/appstream-glib,ikeydoherty/appstream-glib