const the PerlIO vtables

Previously the PerlIO vtables were const only on PERL_GLOBAL_STRUCT_PRIVATE
builds, which was created for former Symbian port and today is rarely used.
Since writing/hooking the PerlIO vtables from XS has never been public API
and is very unlikely to be occuring on CPAN, make then const so they will
be shared between perl processes.

b4 VC2008 x64 perl521.dll .rdata section 0x7293C bytes, .data 0x34B0 bytes
after .rdata 0x7321C .data 0x2BD0