toke.c/MAD: silence a couple of warnings

A couple of vars are set but not used under -Dmad.
The easiest way to silence the warnings is to use PERL_UNUSED_VAR()
rather than try to #ifdef away all the places the vars appear under mad.
dump.c: avoid compiler warning under -Dmad

this fix was already applied to the non-MAD branch; apply it to
the similar MAD / xml-dump code.

dump.c:2663:57: warning: comparison of constant 85 with expression of type
      'svtype' is always false [-Wtautological-constant-out-of-range-compare]
    else if (sv == (const SV *)0x55555555 || SvTYPE(sv) == 'U') {