gtkdochelper: print warnings
gtkdochelper: print warnings

The GTK-Doc commands output was never printed, except in case of error.
So there was no way to see the warnings.

https://github.com/mesonbuild/meson/issues/2396

MathieuDuponchelle/meson,QuLogic/meson,MathieuDuponchelle/meson,mesonbuild/meson,MathieuDuponchelle/meson,jpakkane/meson,jeandet/meson,jpakkane/meson,becm/meson,jpakkane/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,becm/meson,thiblahute/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,thiblahute/meson,pexip/meson,jeandet/meson,becm/meson,becm/meson,pexip/meson,QuLogic/meson,QuLogic/meson,becm/meson,jeandet/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,jeandet/meson,QuLogic/meson,jpakkane/meson,thiblahute/meson,mesonbuild/meson,becm/meson,pexip/meson,becm/meson,becm/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,MathieuDuponchelle/meson,jpakkane/meson,QuLogic/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,thiblahute/meson,jpakkane/meson,jeandet/meson,thiblahute/meson,jeandet/meson,thiblahute/meson,pexip/meson,thiblahute/meson,pexip/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,QuLogic/meson,becm/meson,jeandet/meson,pexip/meson,thiblahute/meson,jeandet/meson,mesonbuild/meson,thiblahute/meson,becm/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,jeandet/meson