clock-vmstate: Add missing END_OF_LIST

Add the missing VMSTATE_END_OF_LIST to vmstate_muldiv

Fixes: 99abcbc7 ("clock: Provide builtin multiplier/divider")
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20220111101934.115028-1-dgilbert@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Luc Michel <luc@lmichel.fr>
Cc: qemu-stable@nongnu.org
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>