[kernel] fix incompatible type error for "__rt_ffs()"
static rt_int16_t rt_scheduler_lock_nest;
extern volatile rt_uint8_t rt_interrupt_nest;
-extern int __rt_ffs(int value);
+extern rt_ubase_t __rt_ffs(rt_ubase_t value);
rt_list_t rt_thread_priority_table[RT_THREAD_PRIORITY_MAX];
struct rt_thread *rt_current_thread;
