overrides: Conditionalize touch override support in Gdk
overrides: Conditionalize touch override support in Gdk

Conditionalize Gdk.EventTouch union discrimination. This fixes the
Gdk overrides to continue working with versions of Gdk less than 3.4.

https://bugzilla.gnome.org/show_bug.cgi?id=747717

GNOME/pygobject,GNOME/pygobject,sfeltman/pygobject,sfeltman/pygobject,pexip/pygobject,GNOME/pygobject,pexip/pygobject,sfeltman/pygobject,pexip/pygobject