Staging: zram: make ZRAM depends on SYSFS

We can not configure zram device without sysfs anyway, so make zram
depends on it.

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>