[MIPS] unwind_stack should return a value ...

And gcc 3.4 doesn't even warn out this, grrr.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/stacktrace.h

@@ -12,6 +12,7 @@ extern unsigned long unwind_stack(struct task_struct *task, unsigned long *sp,
 static inline unsigned long unwind_stack(struct task_struct *task,
 	unsigned long *sp, unsigned long pc, unsigned long *ra)
 {
+	return 0;
 }
 #endif