gtkaboutdialog: don't break ABI in GtkLicense enum

gtkaboutdialog: don't break ABI in GtkLicense enum

As the elements in the enum are not explicitly numbered, inserting an
element between some other elements shall change the values of elements
after the insertion. So append the new element at the end.

Anyway, no code should rely on the position of an element in an enum.

https://bugzilla.gnome.org/show_bug.cgi?id=763850