[S390] Fix stacktrace compile bug.

Add missing module.h include to fix this:

  CC      arch/s390/kernel/stacktrace.o
arch/s390/kernel/stacktrace.c:84: warning: data definition has no type or storage class
arch/s390/kernel/stacktrace.c:84: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL_GPL'
arch/s390/kernel/stacktrace.c:84: warning: parameter names (without types) in function declaration
arch/s390/kernel/stacktrace.c:97: warning: data definition has no type or storage class
arch/s390/kernel/stacktrace.c:97: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL_GPL'
arch/s390/kernel/stacktrace.c:97: warning: parameter names (without types) in function declaration

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

arch/s390/kernel/stacktrace.c

@@ -10,6 +10,7 @@
 #include <linux/sched.h>
 #include <linux/stacktrace.h>
 #include <linux/kallsyms.h>
+#include <linux/module.h>
 
 static unsigned long save_context_stack(struct stack_trace *trace,
 					unsigned long sp,