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>
diff --git a/arch/arm/kernel/stacktrace.c b/arch/arm/kernel/stacktrace.c
index ae31deb..6b3ffde 100644
--- a/arch/arm/kernel/stacktrace.c
+++ b/arch/arm/kernel/stacktrace.c
@@ -66,4 +66,5 @@
 
 	walk_stackframe(fp, base, base + THREAD_SIZE, save_trace, &data);
 }
+EXPORT_SYMBOL_GPL(save_stack_trace);
 #endif