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
7cd9765f9; reported by Bruce Momjian.
Discussion: https://postgr.es/m/E1l33wr-0005sJ-9n@gemulon.postgresql.org