build: fix syntax-check error.

* cfg.mk: Exclude names-refs, it includes a "double" if (end of first
line, first of second line below).

test.y:43.12-44.59:  symbol not found in production: if
 if-stmt-a: IF expr[cond] THEN stmt.list[then] ELSE stmt.list[else] FI
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^