Add an ORDER BY to one more SELECT DISTINCT test case, per buildfarm results.

src/test/regress/expected/select_distinct.out

@@ -4,7 +4,7 @@
 --
 -- awk '{print $3;}' onek.data | sort -n | uniq
 --
-SELECT DISTINCT two FROM tmp;
+SELECT DISTINCT two FROM tmp ORDER BY 1;
  two 
 -----
    0

src/test/regress/sql/select_distinct.sql

@@ -5,7 +5,7 @@
 --
 -- awk '{print $3;}' onek.data | sort -n | uniq
 --
-SELECT DISTINCT two FROM tmp;
+SELECT DISTINCT two FROM tmp ORDER BY 1;
 
 --
 -- awk '{print $5;}' onek.data | sort -n | uniq