Fix the ext/IO/t/io_sock.t failure when built with VC++ 2005

Fix the ext/IO/t/io_sock.t failure when built with VC++ 2005

Unknown to me win32/win32.h was defining USE_FIXED_OSFHANDLE, which
arranged for a black magic fix to MSVCRT.DLL's _open_osfhandle() to
be used. It seems that this is inappropriate for VC++ versions later
than 6.x, since they don't use that DLL: simply not defining that
symbol makes the io_sock.t failure go away.

(Compare change #29233, which similarly disabled the fix to
MSVCRT.DLL's read() for VC++ versions later than 6.x.)
p4raw-link: @29233 on //depot/perl: 46e77f111828d72136c91f0837803182535da01d

p4raw-id: //depot/perl@31271