s390/hvc_iucv: fix broken Kconfig select statement

Select statements in Kconfig do not necessarily enable all required
dependencies and can lead to broken configs. This is also the case
for the "select IUCV" statement within HVC_IUCV:

warning: (HVC_IUCV) selects IUCV which has unmet direct
         dependencies (NET && S390)

Just add the missing "depends on NET" to avoid broken configs.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>