Add copyright notice to vacuum_horizon_floor.pl test.

Fix oversight in commit 303ba0573, which was backpatched through 14.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/CAD21AoBeFdTJcwUfUYPcEgONab3TS6i1PB9S5cSXcBAmdAdQKw%40mail.gmail.com
Backpatch-through: 14

src/test/recovery/t/043_vacuum_horizon_floor.pl

@@ -1,10 +1,12 @@
+# Copyright (c) 2025, PostgreSQL Global Development Group
+#
+# Test that vacuum prunes away all dead tuples killed before OldestXmin
+
 use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use Test::More;
 
-# Test that vacuum prunes away all dead tuples killed before OldestXmin
-
 # Set up nodes
 my $node_primary = PostgreSQL::Test::Cluster->new('primary');
 $node_primary->init(allows_streaming => 'physical');