Preparing for GLib 3.0: g_variant_get_string() to take one argument

[vmagnin]

In https://gitlab.gnome.org/GNOME/glib/-/milestones/2#tab-issues
we can see in the future GLib 3.0 the API break Change g_variant_get_string() to take one argument

In gtk-fortran (gtk4), the only concerned file is:

$ ack g_variant_get_string
examples/menubar.f90
42:             & g_variant_new_string, g_variant_get_string, g_variant_type_new, &
82:    param_string = g_variant_get_string (param, c_null_ptr)