Buildfarm members that lack both HAVE_LOCALE_T and USE_ICU have been
complaining about pg_newlocale_from_collation's collcollate variable.
This is evidently fallout from commit 7d1297df, which removed the
only usage outside those two #ifdef'd code paths.  Mark the variable
pg_attribute_unused(), like its sibling collctype, which has been that
way for a long time.