mkosi: remove intltool from deps

intltool is unnecessary since c8121792.
But we still need gettext, because we use msgformat to generate .mo files.
(I tried to make meson depend on it, at least in Fedora:
 https://bugzilla.redhat.com/show_bug.cgi?id=1545926, but that didn't fly,
 and in Debian at least meson also has no dependency on the tools used by
 its i18n module, so we need to add our own dependency anyway.)

Closes #8192.