[group] Fix copy & paste error from 857b0239e (Use FixedLayout inside Group)
[group] Fix copy & paste error from 857b0239e (Use FixedLayout inside Group)

clutter_group_get_preferred_height was calling
clutter_layout_manager_get_preferred_width which e.g.  broke
test-actor-clone.

Distrotech/clutter,dlespiau/clutter,djdeath/clutter,dlespiau/clutter,dlespiau/clutter,djdeath/clutter,djdeath/clutter-android,GNOME/clutter,djdeath/clutter,ebassi/clutter,heysion/clutter-clone,nobled/clutter,jigpu/clutter,rkudiyarov/clutter_osx,Distrotech/clutter,jigpu/clutter,djdeath/clutter-android,heysion/clutter-clone,kerrickstaley/clutter-vala,GNOME/clutter,Distrotech/clutter,rkudiyarov/clutter_osx,djdeath/clutter,Distrotech/clutter,rkudiyarov/clutter_osx,jigpu/clutter,djdeath/clutter-multithreaded,nobled/clutter,djdeath/clutter-android,collects/clutter,spatulasnout/clutter,nobled/clutter,Distrotech/clutter,djdeath/clutter,ebassi/clutter,spatulasnout/clutter,kerrickstaley/clutter-vala,djdeath/clutter-multithreaded,collects/clutter,collects/clutter,djdeath/clutter-android,djdeath/clutter,GNOME/clutter,heysion/clutter-clone,spatulasnout/clutter,GNOME/clutter,djdeath/clutter-android,ebassi/clutter,jigpu/clutter,kerrickstaley/clutter-vala,djdeath/clutter-multithreaded,collects/clutter,GNOME/clutter,spatulasnout/clutter,djdeath/clutter-multithreaded,kerrickstaley/clutter-vala,jigpu/clutter,Distrotech/clutter,heysion/clutter-clone,jigpu/clutter,ebassi/clutter,heysion/clutter-clone,rkudiyarov/clutter_osx,spatulasnout/clutter,collects/clutter,ebassi/clutter