[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>

arch/s390/Kconfig

@@ -572,6 +572,7 @@ config KEXEC
 config CRASH_DUMP
 	bool "kernel crash dumps"
 	depends on 64BIT
+	select KEXEC
 	help
 	  Generate crash dump after being started by kexec.
 	  Crash dump kernels are loaded in the main kernel with kexec-tools