gtk-vnc: build with GTK3 (squash into virt-manager update)

virt-manager is the only user of this package, and as of virt-manager
1.0.0 it requires gtk-vnc with GTK3 support. The build now needs vala
too.
Revert "gtk-vnc: build with GTK3 (squash into virt-manager update)"

This reverts commit 4e1ed7ec52f79ebbda04265f0fb32a61151f29b6.

virt-manager complains that it wants GTK 2.0 but GTK 3.0 is already
loaded.