Use correct function declaration for yyerror

[dg612_cam.ac.uk]

Summary

According to the POSIX standard yyerror should return int.

[kib]

POSIX standard states that yyerror() function supplied to the parser should have the int return type.
So I believe that instead of this patch, two other places in usr.sbin/config should be patched, that do use void.

[dg612_cam.ac.uk]

Updated to return int as suggested by @kib.

[kib]

usr.sbin/config/config.y

307

return (0);

[dg612_cam.ac.uk]

Added parentheses around the returned value.

[imp]

I can land this too, it looks perfect now.