build: Make g-ir-scanner warnings fatal when werror is set

This should help catching problems with introspection in CI.

Adapted from Sebastian Wick's corresponding mutter change
in https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3528.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3121>

subprojects/shew/src/meson.build

@@ -23,6 +23,7 @@ libshew_gir = gnome.generate_gir(libshew,
   namespace: 'Shew',
   includes: ['Gdk-4.0', 'Gtk-4.0'],
   extra_args: ['--quiet'],
+  fatal_warnings: get_option('werror'),
   install_dir_gir: girdir,
   install_dir_typelib: typelibdir,
   install: true,