Remove stray semicolon at global scope

The Sun Studio compiler complains about an empty declaration here.

Note for future historians:  This does not mean that this compiler is
still of current interest for anyone using PostgreSQL.  But we can let
this small fix be its parting gift.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/a0f817ee-fb86-483a-8a14-b6f7f5991b6e%40eisentraut.org

src/backend/replication/logical/slotsync.c

@@ -1337,7 +1337,7 @@ reset_syncing_flag()
 	SpinLockRelease(&SlotSyncCtx->mutex);
 
 	syncing_slots = false;
-};
+}
 
 /*
  * The main loop of our worker process.