libindicator fails to build against glib 2.55.x

[DimStar]

GNOME 3.28 is going to be release shortly (next week) and will be based on GLIB 2.56.0

With the latest available rc, libindicator fails to build with this error:

[ 86s] CC libindicator3_la-indicator-object-enum-types.lo
[ 86s] ../../libindicator/indicator-ng.c: In function 'indicator_ng_init':
[ 86s] ../../libindicator/indicator-ng.c:690:21: error: assignment from incompatible pointer type [-Werror=incompatible-pointer-types]
[ 86s] self->entry.label = g_object_ref_sink (gtk_label_new (NULL));
[ 86s] ^
[ 86s] ../../libindicator/indicator-ng.c:691:21: error: assignment from incompatible pointer type [-Werror=incompatible-pointer-types]
[ 86s] self->entry.image = g_object_ref_sink (gtk_image_new ());
[ 86s] ^
[ 86s] ../../libindicator/indicator-ng.c:693:20: error: assignment from incompatible pointer type [-Werror=incompatible-pointer-types]
[ 86s] self->entry.menu = g_object_ref_sink (gtk_menu_new ());
[ 86s] ^
[ 87s] cc1: all warnings being treated as errors