tile: export a handful of symbols appropriately

This was shown up by running with "allmodconfig".  I used
EXPORT_SYMBOL() to match existing conventions in files that
were already exporting symbols, or that were exported that way
by other architectures, and otherwise EXPORT_SYMBOL_GPL().

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
diff --git a/arch/tile/kernel/stack.c b/arch/tile/kernel/stack.c
index ba2b36c..ed258b8 100644
--- a/arch/tile/kernel/stack.c
+++ b/arch/tile/kernel/stack.c
@@ -484,6 +484,7 @@
 {
 	save_stack_trace_tsk(NULL, trace);
 }
+EXPORT_SYMBOL_GPL(save_stack_trace);
 
 #endif