Fix thinko in perlsdio.h.

Fix thinko in perlsdio.h.

In 04ce8649990 I inadvertently created conflicting definitions for
PerlIO_ungetc on the non-VMS branch of an #ifdef.  This broke a
NetWare build that was still based on the stdio layer.  See
<http://nntp.perl.org/group/perl.perl5.porters/228427>.  Even
though stdio is no longer supported, we really ought to ship
something that compiles up until the point where we remove it,
which will hopefully be soon for perlsdio.h.

N.B.  This commit is not sufficient to get stdio as the base
layer working again.  Notably XS::APITest is broken for unrelated
reasons.