[MIPS] Fix build error if CONFIG_KALLSYMS is undefined.

  CC      arch/mips/kernel/traps.o
arch/mips/kernel/traps.c: In function 'show_backtrace':
arch/mips/kernel/traps.c:110: warning: unused variable 'ra'
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/stacktrace.h

@@ -9,7 +9,10 @@ extern unsigned long unwind_stack(struct task_struct *task, unsigned long *sp,
 				  unsigned long pc, unsigned long *ra);
 #else
 #define raw_show_trace 1
-#define unwind_stack(task, sp, pc, ra)	0
+static inline unsigned long unwind_stack(struct task_struct *task,
+	unsigned long *sp, unsigned long pc, unsigned long *ra)
+{
+}
 #endif
 
 static __always_inline void prepare_frametrace(struct pt_regs *regs)