ParseXS.pm: Only inc PL_amagic_generation before 5.9

ParseXS.pm: Only inc PL_amagic_generation before 5.9

Originally, overload would not oven be checked for if
amagic_generation was 0, so it was necessary to do
PL_amagic_generation++, in case this was the first class to have over-
loading.  Ever since perl-5.8.0-87-g439cb1c, PL_amagic_generation++
has been unnecessary, since the boot code for version objects incre-
ments it.  Furthermore, newXS was already doing PL_sub_generation++
before that, and now does mro_method_changed_in.  The code for check-
ing the staleness of the overload tables has always checked
sub_generation (and, later, the stash-specific generation numbers).