glibmm/objectbase.h: Mark _gobject_cppinstance_already_deleted()
glibmm/objectbase.h: Mark _gobject_cppinstance_already_deleted()

...with GLIBMM_API.  This is needed for building gtkmm, with Visual
Studio.

GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm,GNOME/glibmm