Put enable_self_join_elimination into postgresql.conf.sample

fc069a3a63 implements Self-Join Elimination (SJE) and provides a new
GUC variable: enable_self_join_elimination.  This commit adds
enable_self_join_elimination to the postgresql.conf.sample, as it was
forgotten in the original commit.

Discussion: https://postgr.es/m/CAHewXN%3D%2Bghd6O6im46q7j2u6c3H6vkXtXmF%3D_v4CfGSnjje8PA%40mail.gmail.com
Author: Tender Wang <tndrwang@gmail.com>

src/backend/utils/misc/postgresql.conf.sample

@@ -427,6 +427,7 @@
 #enable_tidscan = on
 #enable_group_by_reordering = on
 #enable_distinct_reordering = on
+#enable_self_join_elimination = on
 
 # - Planner Cost Constants -