examples: gl/gtk does not build on macOS due to GTK API changes
Thanks to https://gitlab.gnome.org/GNOME/gtk/-/issues/1737 ,
tests/examples/gl/gtk/gstgtk.c is failing to compile on macOS with newer GTK.
It seems like the API breakage is considered OK by the GTK developers, and the solution is to use a new header with newer GTK versions according to https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/3450 .
This is in no release yet.