build: Remove incorrect i18n.merge_file argument

The positional argument was being silently ignored until meson 0.60.0 where
it fails with "ERROR: Function does not take positional arguments".

Related: https://github.com/mesonbuild/meson/issues/9441
Fixes: https://gitlab.gnome.org/GNOME/gvfs/-/issues/599

(cherry picked from commit 17a067b9)