microblaze: Fix sparse warning - unwind.c

Warning log:
CHECK   arch/microblaze/kernel/unwind.c
arch/microblaze/kernel/unwind.c:186:6: warning: symbol 'microblaze_unwind_inner' was not declared. Should it be static?
Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/kernel/unwind.c

@@ -183,7 +183,7 @@ static inline void unwind_trap(struct task_struct *task, unsigned long pc,
  * @trace : Where to store stack backtrace (PC values).
  *	    NULL == print backtrace to kernel log
  */
-void microblaze_unwind_inner(struct task_struct *task,
+static void microblaze_unwind_inner(struct task_struct *task,
 			     unsigned long pc, unsigned long fp,
 			     unsigned long leaf_return,
 			     struct stack_trace *trace)