meson: move i18n module import to only when it is used

When translations are disabled, it's not necessary to `import('i18n')`
and do nothing with it. Also, importing it is (slightly) slow as Meson
needs to load another implementation file from disk, so why bother with
that work?

More particularly, muon does not yet implement this module and fails to
setup. Since there's already an option to disable using it, it makes
sense to let that option completely skip the not-implemented
functionality and actually succeed.

po/meson.build

@@ -1,9 +1,9 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
-i18n = import('i18n')
 want_translations = get_option('translations')
 
 if want_translations
+        i18n = import('i18n')
         i18n.gettext(meson.project_name(),
                      preset : 'glib',
                      data_dirs : '.')