gdb: use dynamic year in update-freebsd.sh

When running update-freebsd.sh on FreeBSD, I see the following
modification in freebsd.xml,

-<!-- Copyright (C) 2009-2023 Free Software Foundation, Inc.
+<!-- Copyright (C) 2009-2020 Free Software Foundation, Inc.

It means that each time, when we running the update-freebsd.sh on
FreeBSD, we have to correct the year of copyright manually. So fix this
issue by using dynamic year.

Tested by regenerating freebsd.xml on FreeBSD/amd64.

Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Simon Marchi <simon.marchi@efficios.com>

gdb/syscalls/update-freebsd.sh

diff --git a/gdb/syscalls/update-freebsd.sh b/gdb/syscalls/update-freebsd.sh
index b5b882e..ce82564 100755
--- a/gdb/syscalls/update-freebsd.sh
+++ b/gdb/syscalls/update-freebsd.sh
@@ -33,10 +33,12 @@
    exit 1
 fi
 
+year=$(date +%Y)
+
 cat > freebsd.xml.tmp <<EOF
 <?xml version="1.0"?> <!-- THIS FILE IS GENERATED -*- buffer-read-only: t -*-  -->
 <!-- vi:set ro: -->
-<!-- Copyright (C) 2009-2020 Free Software Foundation, Inc.
+<!-- Copyright (C) 2009-$year Free Software Foundation, Inc.
 
      Copying and distribution of this file, with or without modification,
      are permitted in any medium without royalty provided the copyright