Nautilus build option change

[Ernestas Kulik <ernestask gnome org>]

Hello, distributors,

Nautilus now builds the image property page, shown in the file property
dialog, as an extension, which causes a change in the build options,
and so `nst_extension` is now `extensions`[1]. The default value
remains unchanged.

[1] - https://gitlab.gnome.org/GNOME/nautilus/blob/cdeedcd354d9d39aaa1a
870228ad40a4066b2486/meson_options.txt

Thanks,
Ernestas Kulik