gnome.genmarshal: Use --output when available
gnome.genmarshal: Use --output when available

This is just cleaner and works around #1417

jeandet/meson,jpakkane/meson,QuLogic/meson,trhd/meson,MathieuDuponchelle/meson,wberrier/meson,mesonbuild/meson,QuLogic/meson,MathieuDuponchelle/meson,trhd/meson,jpakkane/meson,ernestask/meson,rhd/meson,MathieuDuponchelle/meson,aaronp24/meson,aaronp24/meson,wberrier/meson,fmuellner/meson,aaronp24/meson,jpakkane/meson,jpakkane/meson,wberrier/meson,pexip/meson,pexip/meson,aaronp24/meson,rhd/meson,thiblahute/meson,thiblahute/meson,trhd/meson,becm/meson,QuLogic/meson,fmuellner/meson,aaronp24/meson,mesonbuild/meson,ernestask/meson,MathieuDuponchelle/meson,fmuellner/meson,pexip/meson,MathieuDuponchelle/meson,trhd/meson,thiblahute/meson,thiblahute/meson,mesonbuild/meson,QuLogic/meson,wberrier/meson,rhd/meson,pexip/meson,thiblahute/meson,pexip/meson,QuLogic/meson,becm/meson,jpakkane/meson,thiblahute/meson,MathieuDuponchelle/meson,ernestask/meson,ernestask/meson,jeandet/meson,becm/meson,MathieuDuponchelle/meson,wberrier/meson,rhd/meson,ernestask/meson,ernestask/meson,mesonbuild/meson,MathieuDuponchelle/meson,pexip/meson,thiblahute/meson,rhd/meson,fmuellner/meson,mesonbuild/meson,pexip/meson,QuLogic/meson,becm/meson,jpakkane/meson,fmuellner/meson,trhd/meson,mesonbuild/meson,mesonbuild/meson,jeandet/meson,QuLogic/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,aaronp24/meson,wberrier/meson,aaronp24/meson,wberrier/meson,trhd/meson,mesonbuild/meson,jeandet/meson,rhd/meson,becm/meson,rhd/meson,QuLogic/meson,thiblahute/meson,becm/meson,thiblahute/meson,pexip/meson,fmuellner/meson,becm/meson,mesonbuild/meson,trhd/meson,trhd/meson,jpakkane/meson,jeandet/meson,becm/meson,fmuellner/meson,pexip/meson,becm/meson,jeandet/meson,fmuellner/meson,ernestask/meson,jeandet/meson,jpakkane/meson,ernestask/meson,fmuellner/meson,trhd/meson,ernestask/meson,aaronp24/meson,mesonbuild/meson,rhd/meson,wberrier/meson,aaronp24/meson,becm/meson,jeandet/meson,QuLogic/meson,jpakkane/meson