Fix a warning

```
../gc.c:2342:25: warning: array subscript is of type 'char' [-Wchar-subscripts]
    GC_ASSERT(size_pools[pool_id].slot_size == slot_size);
                        ^~~~~~~~
```

gc.c

@@ -2331,7 +2331,7 @@ rb_gc_obj_slot_size(VALUE obj)
 }
 
 static inline size_t
-size_pool_slot_size(char pool_id)
+size_pool_slot_size(unsigned char pool_id)
 {
     GC_ASSERT(pool_id < SIZE_POOL_COUNT);