fs/iso9660: use if ... endif block instead of depends on

In preparation to the addition of numerous additional options to the
iso9660 filesystem logic, use a if ... endif block instead of a
depends on for the only option that currently exists.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>