nds32: Fix the symbols undefined issue by exporting them.

It broke the 'allmodconfig' build.
  LD      vmlinux
  SYSMAP  System.map
  Building modules, stage 2.
  MODPOST 5028 modules
ERROR: "flush_dcache_page" [net/sunrpc/xprtrdma/rpcrdma.ko] undefined!
ERROR: "empty_zero_page" [net/ceph/libceph.ko] undefined!
ERROR: "save_stack_trace" [kernel/backtracetest.ko] undefined!
ERROR: "clear_page" [fs/ocfs2/dlm/ocfs2_dlm.ko] undefined!
ERROR: "copy_page" [fs/nilfs2/nilfs2.ko] undefined!
...

Signed-off-by: Greentime Hu <greentime@andestech.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

arch/nds32/kernel/stacktrace.c

@@ -9,6 +9,7 @@ void save_stack_trace(struct stack_trace *trace)
 {
 	save_stack_trace_tsk(current, trace);
 }
+EXPORT_SYMBOL_GPL(save_stack_trace);
 
 void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 {
@@ -45,3 +46,4 @@ void save_stack_trace_tsk(struct task_struct *tsk, struct stack_trace *trace)
 		fpn = (unsigned long *)fpp;
 	}
 }
+EXPORT_SYMBOL_GPL(save_stack_trace_tsk);