stacktrace: fix modular build, export print_stack_trace and save_stack_trace

fix:

ERROR: "print_stack_trace" [kernel/backtracetest.ko] undefined!
ERROR: "save_stack_trace" [kernel/backtracetest.ko] undefined!
Signed-off-by: NIngo Molnar <mingo@elte.hu>

and fix:

  Building modules, stage 2.
  MODPOST 376 modules
ERROR: "print_stack_trace" [kernel/backtracetest.ko] undefined!
make[1]: *** [__modpost] Error 1
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/stacktrace.c

@@ -74,6 +74,7 @@ void save_stack_trace(struct stack_trace *trace)
 	if (trace->nr_entries < trace->max_entries)
 		trace->entries[trace->nr_entries++] = ULONG_MAX;
 }
+EXPORT_SYMBOL_GPL(save_stack_trace);
 
 void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 {
@@ -81,3 +82,4 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 	if (trace->nr_entries < trace->max_entries)
 		trace->entries[trace->nr_entries++] = ULONG_MAX;
 }
+EXPORT_SYMBOL_GPL(save_stack_trace_tsk);

kernel/stacktrace.c

@@ -6,6 +6,7 @@
  *  Copyright (C) 2006 Red Hat, Inc., Ingo Molnar <mingo@redhat.com>
  */
 #include <linux/sched.h>
+#include <linux/module.h>
 #include <linux/kallsyms.h>
 #include <linux/stacktrace.h>
 
@@ -21,4 +22,5 @@ void print_stack_trace(struct stack_trace *trace, int spaces)
 		print_ip_sym(trace->entries[i]);
 	}
 }
+EXPORT_SYMBOL_GPL(print_stack_trace);