Suppress bison warning in ecpg grammar.

opt_distinct_clause is only used in PLpgSQL_Expr, which ecpg
ignores, so it needs to ignore opt_distinct_clause too.

My oversight in 7cd9765; reported by Bruce Momjian.

Discussion: https://postgr.es/m/E1l33wr-0005sJ-9n@gemulon.postgresql.org

Author: tglsfdc

src/interfaces/ecpg/preproc/parse.pl

@@ -71,6 +71,7 @@
 	'type_function_name' => 'ignore',
 	'ColLabel'           => 'ignore',
 	'Sconst'             => 'ignore',
+	'opt_distinct_clause' => 'ignore',
 	'PLpgSQL_Expr'       => 'ignore',
 	'PLAssignStmt'       => 'ignore',
 	'plassign_target'    => 'ignore',