[S390] Kconfig: Select CONFIG_KEXEC for CONFIG_CRASH_DUMP

The kdump infrastructure is built on top of kexec. Therefore
CONFIG_KEXEC has to be enabled when CONFIG_CRASH_DUMP is selected.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>