s390/zfcpdump: make zfcpdump depend on 64BIT

Get rid of this link error:

arch/s390/built-in.o: In function `smp_prepare_cpus':
(.init.text+0x301e): undefined reference to `dump_save_area_create'

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