s390/Kconfig: select 'TTY' when 'S390_GUEST' is enabled

'VIRTIO_CONSOLE' depends on 'TTY', so need to select 'TTY' explicitly
before selecting 'VIRTIO_CONSOLE'.

Otherwise randconfig can generate a config file which enables
'VIRTIO_CONSOLE' but has 'TTY' disabled.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>