s390/kdump: get rid of compile warning

Move the CONFIG_CRASH_DUMP ifdef to get rid of this:

arch/s390/kernel/machine_kexec.c:146:22: warning: 'do_start_kdump' defined but not used [-Wunused-function]

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