stacktrace: export save_stack_trace[_tsk]

Andrew Morton reported this against linux-next:

ERROR: ".save_stack_trace" [tests/backtracetest.ko] undefined!

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/avr32/kernel/stacktrace.c

@@ -51,3 +51,4 @@ void save_stack_trace(struct stack_trace *trace)
 		fp = frame->fp;
 	}
 }
+EXPORT_SYMBOL_GPL(save_stack_trace);