GAsync: Declare functions for abstract async methods
GAsync: Declare functions for abstract async methods

Fixes bug 598266.

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