[S390] s390: Fix build for !CONFIG_S390_GUEST + CONFIG_VIRTIO_CONSOLE

The s390 kernel does not compile if virtio console is enabled, but guest
support is disabled:

  LD      .tmp_vmlinux1
arch/s390/kernel/built-in.o: In function `setup_arch':
/space/linux-2.5/arch/s390/kernel/setup.c:773: undefined reference to
`s390_virtio_console_init'

The fix is related to
commit 99e65c92
Author: Christian Borntraeger <borntraeger@de.ibm.com>
Date:   Fri Jul 25 15:50:04 2008 +0200
    KVM: s390: Fix guest kconfig

Which changed the build process to build kvm_virtio.c only if CONFIG_S390_GUEST
is set. We must ifdef the prototype in the header file accordingly.

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