gtask: Add guards for public functions
gtask: Add guards for public functions

https://bugzilla.gnome.org/show_bug.cgi?id=769745

endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib