hw/rtc/mc146818: Add missing dependency on ISA Bus

The MC146818 sits on an ISA bus. Since it can not be used
without it, select ISA in the Kconfig.

Fixes: 82f51817 ("kconfig: introduce kconfig files", 2019-03-07)
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20191224111628.3551-1-philmd@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>