headerbar: remove G_PARAM_CONSTRUCT from custom-title
headerbar: remove G_PARAM_CONSTRUCT from custom-title

This fixes the ability to sublcass GtkHeaderBar from a UI template which
contains a custom <child type="title"> element.

https://bugzilla.gnome.org/show_bug.cgi?id=757544

grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita