pgstat: Mention pgstat_replslot.c in pgstat.c.

Oversight, by me, in commit 5891c7a8ed.

Author: "Drouvot, Bertrand" <bdrouvot@amazon.com>
Discussion: https://postgr.es/m/bd58e027-6598-57a2-679b-d576d17bfaa9@amazon.com

src/backend/utils/activity/pgstat.c

@@ -73,6 +73,7 @@
  * - pgstat_database.c
  * - pgstat_function.c
  * - pgstat_relation.c
+ * - pgstat_replslot.c
  * - pgstat_slru.c
  * - pgstat_subscription.c
  * - pgstat_wal.c