Add a missing include to enums.ccg.
Add a missing include to enums.ccg.

* gtk/src/enums.ccg: Add #include <gtkmm/settings.h>.

GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm,GNOME/gtkmm,GNOME/gtkmm,johne53/MB3Gtkmm