translations: Run msgmerge when downloading translations

The latest translations in the translationproject URL need to be
merged in using msgmerge for the po files to be correctly updated,
otherwise we may end up getting odd results, such as the previous
translations update.  This patch adds another step to the
update-translations Makefile target which does a msgmerge of the
downloaded po file with libc.pot and then uses that as the final
result.