plperl_helpers.h fix for clang

clang warns about that newish SvREFCNT_inc(sv) call in plperl_helpers.h
about an unused return value, because the macro expansion of
SvREFCNT_inc(sv) returns sv. The merit of that warning might be
debatable, but it seems easy to fix by using SvREFCNT_inc_void(sv)
instead.

And we could use SvREFCNT_inc_simple_void(sv), since sv doesn't have any
side effects, but that's optional.

Any concerns?