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>

arch/s390/kernel/machine_kexec.c

@@ -141,7 +141,6 @@ static noinline void __machine_kdump(void *image)
 	 */
 	store_status(__do_machine_kdump, image);
 }
-#endif
 
 static unsigned long do_start_kdump(unsigned long addr)
 {
@@ -155,6 +154,8 @@ static unsigned long do_start_kdump(unsigned long addr)
 	return rc;
 }
 
+#endif /* CONFIG_CRASH_DUMP */
+
 /*
  * Check if kdump checksums are valid: We call purgatory with parameter "0"
  */