Also build PerlIO_get_layers with -Dusesfio

Also build PerlIO_get_layers with -Dusesfio

This extends patch 5fef3b4a7f0c97d3 from April 2003 from fixing the
stdio-only build to also fix the sfio build, which was broken when
PerlIO_get_layers was added (2 commits previously, 39f7a87036eb8d13).
Without this fix, perl would refuse to build with -Dusesfio.

Given that 39f7a87036eb8d13 was merged to maint-5.8, this means that the
-Dusesfio build was broken for v5.8.1 and later on that branch, as well as
for v5.10.0 and all subsequent releases. Given that in the past 10 years,
no-one has reported this, it's safe to assume that no-one considers sfio
support to be important.