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);
                        ^~~~~~~~
```

1 files changed, 1 insertions, 1 deletions

diff --git a/gc.c b/gc.c
index bd3915fb47..b5d5df692b 100644
--- a/gc.c
+++ b/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);