Add cast to `mrb_int` in `mrb_fixnum()` when MRB_WORD_BOXING.
Without this cast, `long int:63` can cause warnings.
@@ -66,7 +66,7 @@ MRB_API mrb_value mrb_word_boxing_float_pool(struct mrb_state*, mrb_float);
#define mrb_ptr(o) (o).value.p
#define mrb_cptr(o) (o).value.vp->p
#define mrb_float(o) (o).value.fp->f
-#define mrb_fixnum(o) (o).value.i
+#define mrb_fixnum(o) ((mrb_int)(o).value.i)
#define mrb_symbol(o) (o).value.sym
static inline enum mrb_vtype
