Without this, the test_slot_timelines modules fails "make installcheck"
because the required feature is not enabled in a stock server.
Per buildfarm
--- /dev/null
+CREATE EXTENSION test_slot_timelines;
+SELECT test_slot_timelines_create_logical_slot('test_slot', 'test_decoding');
+ERROR: replication slots can only be used if max_replication_slots > 0
+SELECT test_slot_timelines_advance_logical_slot('test_slot', txid_current(), txid_current(), pg_current_xlog_location(), pg_current_xlog_location());
+ERROR: replication slots can only be used if max_replication_slots > 0
+SELECT pg_drop_replication_slot('test_slot');
+ERROR: replication slots can only be used if max_replication_slots > 0