[S390] Fix gcc 4.6.0 compile warning

With gcc 4.6.0 we get a false compile warning:

 arch/s390/kernel/setup.c: In function 'setup_arch':
 arch/s390/kernel/setup.c:767:3: warning: 'msg' may be used
            uninitialized in this function [-Wuninitialized]
 arch/s390/kernel/setup.c:753:8: note: 'msg' was declared here

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

arch/s390/kernel/setup.c

@@ -747,7 +747,7 @@ static void __init reserve_crashkernel(void)
 {
 #ifdef CONFIG_CRASH_DUMP
 	unsigned long long crash_base, crash_size;
-	char *msg;
+	char *msg = NULL;
 	int rc;
 
 	rc = parse_crashkernel(boot_command_line, memory_end, &crash_size,