[docs] Add Interval get_type function
[docs] Add Interval get_type function

The get_type() function for ClutterInterval is missing from the
known GObject types, so gtk-doc doesn't know that it has to
introspect it for hierarchy, properties and signals.

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