codegen: Fix support for g_boxed_copy
codegen: Fix support for g_boxed_copy

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