fs/iso9660: fix iso9660 support

Similar to the "fs/initramfs: fix initramfs support" commit the same
problem applies to iso9660 in a different way. By adding iso9660 to
TARGETS it gets called before target-finalize with obvious consequences.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>