"could not identify an equality operator for type hll" in GROUP BY

When I use a column with `hll` type as a field in `GROUP BY` clause, I get the error:
```
ERROR: could not identify an equality operator for type hll
```

Here's a small repro:
```
CREATE TABLE tmp (int num, set hll);
INSERT INTO tmp VALUES (1, hll_empty());
INSERT INTO tmp VALUES (2, hll_empty());
INSERT INTO tmp VALUES (3, hll_empty());
INSERT INTO tmp VALUES (4, hll_empty());
SELECT SUM(num), set FROM tmp GROUP BY set;
```
While I expect it to result in a row consisting of 10 and empty `hll` value, I get the error above.

**postgresql-hll Version**: master (77aa0fe)
**PostgreSQL Version:** 11.5
**OS Version:** Debian 10