readline: fix the attempted_completion_function binding
readline: fix the attempted_completion_function binding

Fixes bug 628966.

Distrotech/vala,spidermonkdust/vala,GNOME/vala,flobrosch/vala-fixes,GNOME/vala,spidermonkdust/vala,kamanashisroy/aroop,rvu95/vala,GNOME/vala,severinh/vala,GNOME/vala,frida/vala,spidermonkdust/vala,jessevdk/vala,frida/vala,jessevdk/vala,tjol/vala-tjol,playya/vala,playya/vala,frida/vala,rvu95/vala,tjol/vala-tjol,frida/vala,frida/vala,flobrosch/vala-fixes,kamanashisroy/aroop,flobrosch/vala-fixes,playya/vala,severinh/vala,severinh/vala,tjol/vala-tjol,jessevdk/vala,kamanashisroy/aroop,Distrotech/vala,rvu95/vala,Distrotech/vala,xdave/pvala