Use type tag for hash code in `ht_hash_func()`
The function corresponding to `ht_hash_func()` was as follows in the days of
khash implementation (before
d78acc7a).
```c
mrb_hash_ht_hash_func(mrb_state *mrb, mrb_value key)
{
enum mrb_vtype t = mrb_type(key);
...
switch (t) {
...
default:
hv = mrb_funcall(mrb, key, "hash", 0);
h = (khint_t)t ^ (khint_t)mrb_fixnum(hv);
break;
}
...
}
```
When switched to the segmented list implementation (
d78acc7a), this function
was changed as follows.
```c
sg_hash_func(mrb_state *mrb, seglist *t, mrb_value key)
{
enum mrb_vtype tt = mrb_type(key);
...
switch (tt) {
...
default:
hv = mrb_funcall(mrb, key, "hash", 0);
h = (size_t)t ^ (size_t)mrb_fixnum(hv);
break;
}
...
}
```
Since the argument `t` was added, the variable for type tag was changed from
`t` to `tt`, but the variable used in the expression of `h` remained `t`.
Probably this is an omission of change, so fixed it.