codegen: Do not declare virtual function for default signal handlers
codegen: Do not declare virtual function for default signal handlers

Fixes bug 615637.

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