po/meson.build: add missing 'ga' language code

Commit bf5ce434db ("l10n: Add full Irish translation (ga.po)", 2025-05-16) added a new translation to git. In a make build, new 'po' files (ga.po in this case) are added to the build automatically using a wildcard pattern. In a meson build you have to add the language code ('ga') to a list explicitly to have it included in the build. In order to include the new translation in the meson build, add the 'ga' language code to the list of translations in the 'po/meson.build' file. Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>