It is recommended to use SSIZE_T for ssize_t on win32, but the commit
that is being used for slirp.wrap uses int.  Update to include the fix
as well as the other bugfix commit "ip: Enforce strict aliasing".

Reported-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>