sql: no privileges required for SHOW CONSTRAINTS

[jseldess]

PR #8070 changed table-based SHOW statements to require any privileges, but we missed SHOW CONSTRAINTS:

jesseseldess@JESSEs-MacBook-Pro:~/src/github.com/cockroachdb/cockroach$ cockroach sql
# Welcome to the cockroach SQL interface.
# All statements must be terminated by a semicolon.
# To exit: CTRL + D.
root@:26257> create table test.a (a int);
CREATE TABLE
root@:26257> grant select on test.a to jsel;
GRANT
root@:26257> show grants on test.a;
+-------+------+------------+
| Table | User | Privileges |
+-------+------+------------+
| a     | jsel | SELECT     |
| a     | root | ALL        |
+-------+------+------------+
(2 rows)
root@:26257> ^D
jesseseldess@JESSEs-MacBook-Pro:~/src/github.com/cockroachdb/cockroach$ cockroach sql --user=maxroach
# Welcome to the cockroach SQL interface.
# All statements must be terminated by a semicolon.
# To exit: CTRL + D.
maxroach@:26257> show constraints from test.a;
+-------+---------+-------------+-----------+---------+
| Table |  Name   |    Type     | Column(s) | Details |
+-------+---------+-------------+-----------+---------+
| a     | primary | PRIMARY KEY | [rowid]   | NULL    |
+-------+---------+-------------+-----------+---------+
(1 row)