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/048_vacuum_horizon_floor.pl

@@ -1,8 +1,5 @@
-use strict;
-use warnings;
-use PostgreSQL::Test::Cluster;
-use Test::More;
-
+# Copyright (c) 2025, PostgreSQL Global Development Group
+#
 # Test that vacuum prunes away all dead tuples killed before OldestXmin
 #
 # This test creates a table on a primary, updates the table to generate dead
@@ -10,6 +7,11 @@ use Test::More;
 # GlobalVisState->maybe_needed on the primary to move backwards and precede
 # the value of OldestXmin set at the beginning of vacuuming the table.
 
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use Test::More;
+
 # Set up nodes
 my $node_primary = PostgreSQL::Test::Cluster->new('primary');
 $node_primary->init(allows_streaming => 'physical');