Don't allow ALTER MATERIALIZED VIEW ADD UNIQUE.

Was accidentally allowed, but not documented and lacked support
for rename or drop once created.

Per report from Noah Misch.

src/backend/commands/tablecmds.c

@@ -3035,7 +3035,7 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			pass = AT_PASS_DROP;
 			break;
 		case AT_AddIndex:		/* ADD INDEX */
-			ATSimplePermissions(rel, ATT_TABLE | ATT_MATVIEW);
+			ATSimplePermissions(rel, ATT_TABLE);
 			/* This command never recurses */
 			/* No command-specific prep needed */
 			pass = AT_PASS_ADD_INDEX;