Suppress "warning: variable 'collcollate' set but not used".

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.

src/backend/utils/adt/pg_locale.c

@@ -1514,7 +1514,7 @@ pg_newlocale_from_collation(Oid collid)
 		/* We haven't computed this yet in this session, so do it */
 		HeapTuple	tp;
 		Form_pg_collation collform;
-		const char *collcollate;
+		const char *collcollate pg_attribute_unused();
 		const char *collctype pg_attribute_unused();
 		struct pg_locale_struct result;
 		pg_locale_t resultp;