codegen: Fix is_reference_counting() result for interfaces
codegen: Fix is_reference_counting() result for interfaces

Fixes a regression introduced by commit 5956bf5074d4508c.

Fixes bug 661620.

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